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is the Journal of the Biological Society, Victoria University of Wellington, New Zealand, and is published three times a year, with the financial assistance of the University Publications fund.
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The purchase of a set of dissecting instruments becomes essential when the student approaches his first year of university work in zoology. This is the time when a little care and thought will lead to the selection of a limited number of instruments which with few additions will serve adequately for many years whether the student continues in zoology or goes on to veterinary science, home science, dentistry, medicine, agriculture, etc.
A minimum of good quality instruments of suitable shapes and sizes, kept in good condition and correctly used, are a prime essential to the aret of dissection, and will prove an economy of time, energy and money. The work of the first year in zoology will include the dissection of animals ranging in size and nature from the small soft-bodied earthworm to the large hard-skinned shark, and other animals such as the frog, and a mammal which might be a rabbit, a white rat or a guinea-pig. A limited number of simple instruments will prove adequate for all this work and for the work of other courses in later years.
It is extremely difficult to describe all the characteristics which enable an experienced person to distinguish instruments of good quality from those which are poor in design, made of cheap materials and with faulty workmanship. Scalpels come in all grades from cheap, case-hardened, all-metal scalpels which are brittle and most difficult to sharpen, to forged scalpels with finely-tempered, high-quality steel blades which sharpen readily to a perfect cutting edge. Avoid stainless steel scalpels which do not take or keep an edge, and those which fail to ring sharply when the scalpel is held lightly by the end of the handle and the tip of the blade is clicked with the thumbnail or tapped on a hard surface such as glass. Avoid scissors which are unduly heavy, thick in the blades or handles, and which fail to cut cleanly at the tip. If you have the opportunity, check a variety of scalpels, forceps, scissors, etc., and try to develop a sense of balance in these instruments. A good all-metal scalpel balances at the middle of the total length; well-designed scissors balance about two-thirds of the length from the tip. Well-made forceps balance at about the mid-point in their length. The double thickness at the hinge in well-designed forceps is thinner than the mid-portion of the arm, and the portion between the hinge and the modelled arm will be the narrowest and the
Most dissection instruments are plated. Good plating is smooth, without pits, cracks or ripples, uniformly and brightly reflective. Chrome plating has a bluish tinge, an oily appearance and a long life in ordinary usage. Nickel plating is silvery and with proper care has a reasonably long life but tends to peel when it breaks. Chrome-plated instruments are preferable but usually somewhat more costly than nickel-plated instruments of similar quality. Even when the plating is worn, a good instrument is fully usable provided it is not allowed to rust.
An adequate dissecting set will include one scalpel; fine and heavy scissors; fine and coarse forceps; two dissecting needles; a probe; a six-inch ruler; dividers; pencil; rubber. Hooks and chains and the blowpipe are described because they are still supplied in many kits which persuades students that these items are necessary, but this is not correct. Hand-lens, pipettes, pins, slides and cover-slips, corded hooks, and other items used from time to time throughout the year are not included here as part of the dissecting kit since these and a sharpening stone are generally supplied to the student in the laboratory as required.
This is a first requirement. Dissection instruments are sharp tools. Edges and points blunt and damage if the instruments are kept loose in a box. Expensive sets are supplied with wooden boxes fitted with racks, but soft cases with pockets or loops for each instrument are less costly and fully satisfactory. Equally suitable is the instrument roll made of heavy linen with pockets or loops and a turn-in flap, much to the pattern of a carpenter's wood-bit roll. A roll can be easily made to the pattern illustrated in Fig. H.
The most costly case or the simplest home-made roll is useless unless instruments are kept in it and in their proper places when not required.
This is much more than an ordinary knife. A good scalpel has a strong sharp point to the blade so that the blade will pass easily in between two sheets of tissue or between other structures. It has a strong dull back to
The sharp edge of the blade should be used only to do the work which the other parts of the scalpel will not perform. This keeps the edge of the blade sharp and saves sharpening. A scalpel with a straight edge is designed for cutting down through thick layers or across solid structures and has little value in general dissection because only a small part of the edge can be used. If the sharp edge is convex there is a greater length of usable edge and as one portion dulls, a change in the angle of attack will allow another portion of the edge to come into use. Keep the edge sharp. Heavy cutting pressure with a dull scalpel can lead to accidents and damage to the dissection or to yourself.
From the above account of the use of a scalpel it can be seen that a single scalpel suitable for a wide range of general work should have a convex blade about l½ in. long by at least ½ in. at its greatest depth. The handle should be at least 4½ in. long so that the scalpel can be reversed and the end of the handle used in dissection without danger to the fingers from the cutting edge.
A good scalpel has a blade which is an acute-angled triangle in section. A deeply hollow-ground blade sharpens easily for light work but will be found too weak for heavy tasks.
Sharpen the scalpel on a fine grade india stone wet with water. Lay the blade flat. Rub the blade lengthwise. Then reverse and rub the other side an equal amount. Take out nicks by using a coarser grade of stone.
Avoid scalpels with replaceable blades. These were not originally designed for general dissection, are too fragile for much heavy work, cannot be sharpened, and usually provide only a short cutting edge.
If anything, these are more generally useful cutting instruments than is the scalpel; but it requires some experience before scissors are freely and properly employed in dissection.
Two types of scissors are required. One is a smaller pair with sharp-
Dissecting Instruments for Zoology pointed blades from 1 in. to 1 ½ in. in length, and the whole instrument of at least 4 in. total length; the other, a larger pair of scissors about 6 in. in total length with strong blades 2 in. long and preferably one blade tapering and sharp-pointed, the other blade being almost parallel-edged and rounded at the end. Obviously, the smaller are used for finer dissection, the larger for heavier work. Scissors with curved blades are of little general use and are employed for trimming away loose strands or edges of sheets of tissue.
It is most important in scissors that they cut to the very tips of the blades when used. Good scissors cut cleanly a piece of paper even less than 1 mm. from the tip, and scissors should be tested this way before purchase.
Dissection of the frog provides a good opportunity for the diversified use of scissors. A scalpel is used in this dissection only when removing the bones forming the roof of the skull. The skin on the frog is divided, as required, by picking up a small fold of the skin in fine forceps, cutting it across just sufficiently to allow entry of the tip of the small scissors and then cutting lengthwise using the greater length of the blade of the scissors. The same scissors are used to cut through the septa which attach the skin to the muscular body-wall. The same technique is used in opening the body cavity; but the heavy scissors are used with the blunt blade within the animal to cut through the pectoral girdle. The blunt end of the blade tends to force aside soft structures with little damage. A sharp pointed blade used in the same way may pierce a structure so that it will be cut or otherwise damaged. Forceps and scissors are used again in dissecting the leg to expose the sciatic nerve passing to the gastrocnemius muscle. Scissors are used to open the pericardium to expose the heart, and to open the heart to examine the chambers and valves.
These are used as an extension of the fingers. They permit the hand to be kept out of the immediate field of the dissection and are used for the grasping of small or large items often for considerable periods of time such as when a structure is kept under tension to assist its dissection, as in exposing a nerve or a blood-vessel passing through connective tissues. Typically a pair of forceps consists of two parallel metal blades or arms fastened permanently at one end as a spring-hinge and ending at the other with a tip of various forms, straight or curved, blunt or sharp.
For general dissection, two pairs of forceps are required, a pointed pair for finer work, a blunt pair for heavy work. In both cases select only forceps which close under light pressure. Forceps having a strong spring hinge rapidly tire the hand. The pressure to close good forceps is so little that it can hardly be appreciated, yet on release the tips spring wide apart, 3/4 in. or more. Good forceps of all sizes have a guide-pin, a small tapering pin mounted on the inner face of one arm and passing through an opening on the opposite arm when the forceps are closed. This small pin should be sturdy since it lines up the tips when closed. It makes the tips meet accurately even under heavy pressure. The inner face of each tip should
Pointed forceps for fine work are held between the thumb and forefinger, or grasped in the hand and so can be used at many angles. For this reason straight forceps are preferred. Forceps with curved tips are more restricted in use and less satisfactory also, because unless a very expensive, well-built pair is obtained, the points tend to separate or twist apart under pressure.
Blunt forceps should be really sturdy, but still only lightly sprung so that when held between thumb and forefinger or grasped in the hand they close easily and open readily to at least 1 in. between the tips. They should be straight, plain enough to be comfortable to hold but with some modelling to fit the hand so that they will not tend to slip if wet and greasy. Some well-built heavy forceps have a low flat-topped stop on the inner face of one arm in addition to the guide-pin. This stop prevents the tips from separating under heavy pressure, but if the stop is placed near the middle of the arm or nearer the tip than the hinge, it is useless.
Because forceps primarily enable the hand to be kept clear from the field of dissection and forceps are used in a variety of positions, both fine and blunt forceps should be at least five to six inches in length.
Avoid forceps which have a strong spring; in which the tips close under light pressure but open at the tips when full pressure is applied; which lack guide-pins; in which the ridging of the tips does not interlock.
Dissection needles are steel needles mounted in the ends of rounded wooden handles. These instruments are more neglected and abused than any other. Simple as these instruments are, they are worthy of respect. They take the place of fine forceps and a scalpel in the minutest dissection such as under the hand-lens. Experienced workers use only a pair of needles to carry out dissections by hand even under the low power of the ordinary microscope.
The needles are used, one in either hand, held between thumb and forefinger and supported on the index finger. One needle is pressed down on to a specimen to hold it in place. The sharp tip of the other needle is pressed lightly on to and drawn along the specimen as though it were a scalpel, and properly used it will cut into the specimen. This is the best way to remove the wings and legs of a small insect such as a fly or a mosquito, to free internal organs in a weta to expose the nervous system, to dissect the head of a preserved small tadpole which is a simpler and clearer dissection than the head of a frog, etc.
Good dissecting needles are less than a millimetre in diameter, sold for ordinary sewing as size 5/9. A needle 1 ½ in. long is of a convenient length. Needles of this size are sufficiently rigid and yet flexible enough to be
In its best form, this is nothing but a piece of german-silver or silver wire about 6 in. long, a good millimetre in diameter, straight or curved as shown (Fig. G), and bluntly rounded at each end. It should be semi-soft, soft enough that it can be pushed gently inside a blood-vessel or duct, and tend to follow the path of the structure. The probe then acts as a guide for scissors which can be used to open the length of the vessel. This is almost a forgotten technique in ordinary dissection and yet it is the simplest way to locate and follow structures such as the anterior cardinal vein in the dogfish, or the ureters in a fatty rat, etc.
The hard-drawn blunt steel wire mounted in a round wooden handle is of little practical use. The instrument in this form has discouraged the use of the probe in dissection; but a good probe is a most useful aid in many dissections.
This was a common instrument in former years when dissections were carried out on fresh material. The blowpipe consisted of a metal tube tapering to about 2 mm. in diameter. This narrow end could be passed into the bladder, into the intestine or other hollow organs and ducts, which could then be inflated to display form and connections. Since most larger dissections are now performed on preserved material, the blowpipe has dropped from use.
These are seen in the large complete dissecting sets such as contain a variety of scalpels, forceps, etc. There is usually a pair in a set. Each consists of three pieces of light chain about 6 in. long joined to a ring and the free ends armed each with a strong sharp hook. They were used in the dissection of large animals, animals of sufficient size that two of the hooks could be separated and driven into the flesh of the animal as anchors for the third hook which could be used to hold an organ or an edge of a flap of body wall, etc., clear from the dissecting field. They are not sufficiently adaptable for use on small animals since they cannot be conveniently
These can hardly be termed dissecting instruments but they have a proper place in any dissecting kit since there is no better way to study a dissection than to prepare a well-proportioned and detailed drawing of the work that has been done. The simplest way to do this is by a mechanical transfer of the proportions of major structures from the dissection to paper. Decide a size for the drawing, a reduction or magnification such as will allow all detail to be conveniently represented. Then take the length and breadth of the various major structures item by item, using the dividers, measuring each on the ruler, reducing or enlarging each according to the scale, and mark out the dimensions in their relative positions on the drawing before attempting the outlines. Outlines should be lightly dotted in first and corrected until a good representation has been obtained. This may appear tedious, but it is the most accurate and the fastest way to obtain a good drawing.
A 6 in. rule is all that is needed; celluloid and plastic are preferable to wood, but wood is adequate. The dividers should have legs at least 6 in. long, the hinge should be firm and capable of being tightened if necessary. A good pencil, kept sharp, is needed. HB or softer grades are unsatisfactory because the drawings rub when assembled into a book or folder, and drawings made with soft pencils smear even to the extent of becoming illegible. Use a 2F or 2H pencil.
Before you buy your rubber, check to see that it does not leave a mark on the paper. A good grade of soft red rubber is adequate.
When we look casually at the sea around us, we may be hardly aware that living concealed beneath the surface, especially in harbour and coastal waters, are millions of tiny organisms— both plant and animal— which together make up the plankton. Each plankton organism is known as a plankter. The plant plankton, or phytoplankton, is composed of unicellular algae, the chief constituents of which are diatoms (encased by a siliceous wall); dinoflagellates (tiny organisms with one transverse and one longitudinal flagellum which ‘screw’ themselves through the water); and even tinier micro-flagellates which are now considered to be of basic importance in providing the bulk of primary marine food production. As Professor C. C. Davis (1955) puts it, the phytoplankters are the producers and the zooplankters the consumers.
Like land plants, the plankton diatoms must have light for their existence. Unlike most land and seashore plants, however, they are forced to float in order to survive, for their existence is possible only in those layers of the ocean through which the sun's rays penetrate. We find that these minute algae are peculiarly adapted to life as single cells. Each cell is capable of existing as an independent unit, even in the many species where the cells are joined together to form long chains. Size is a disadvantage for phytoplankters. A small object will float more readily if it possesses a larger surface area in proportion to its total volume. This will increase resistance through friction and also present the largest possible surface to the energy-giving light from the sun. Several distinctive types of cell construction to assist with flotation have been classified. These are the bladder type (e.g. Coscinodiscus), the ribbon type (e.g. Streptotheca), the hair type (e.g. Rhizoselenia) and the branching type (e.g. Chaetoceros - Plate II, Figs. 1, 5-7, 8-10, 11).
Those species which flourish away from any land mass are said to be oceanic while those inhabiting coastal waters are known as neritic. In general, oceanic diatoms cannot reproduce in coastal waters and hence their occurrence in the neritic zone is limited to one short-lived generation. Diatoms growing attached to a substrate in the littoral or sublittoral regions may break off, float upwards and be collected along with other pelagic (free-floating) forms. Such plants are called tychopelagic.
Quite apart from their importance in the food chain. diatoms play a fundamental role in the world's economy through their contribution to ocean floor deposits which, under great pressure, have become reservoirs of ‘mineral’ oil. Within limits, these organisms can provide physical oceanographers with valuable information about movement and mixing of
Professor A. C. Hardy (1956) has very aptly described a microscopic view of living diatoms. They look to him like ‘crystal caskets filled with jewels as the strands of sparkling protoplasm and groups of amber chloroplasts catch the light’.
Diatoms are among the most cosmopolitan of plant groups, owing to the much more continuous nature of the environment of the sea in comparison with terrestrial habitats. All the same, local differences in species composition are quite to be expected, and until more is known about year-round seasonal variation in all latitudes, it is difficult to generalise on this topic. It has been stated that production of marine phytoplankton in the arctic and antarctic greatly exceeds that in tropical waters (e.g. Hart 1934). Davis (1955) points out, however, that most of the sampling in high-latitude waters has taken place during the summer by ‘hit and run’ expeditions. At such a time the following conditions encourage tremendous productivity: nearly twenty-four hours of light for photosynthesis, weak thermal stratification, plus a super-abundance of nutrient salts from deep upwelling water and from melted ice. It is obvious, however, that total annual production of plant growth cannot be adequately assessed from summer analyses alone.
In the open Pacific, Graham (1941) found that in regions where upwelling of nutrient-rich water occurred, production was greater in tropical than in average temperate waters. Further, Allen (1939) found as many as 500,000 diatoms in a litre of sea-water off the Pacific coast of Panama— a high figure for any latitude. It would seem, therefore, that each locality must be studied in conjunction with the variable physical properties of the local marine environment before broad generalisations can be made on geographic distribution.
In temperate waters, one of the most absorbing problems presented by the phytoplankton is that of sudden appearances and disappearances of one or several species in tremendous quantities. This phenomenon is known as a ‘bloom’. Although the precise factors which trigger off a bloom are not yet fully understood, the general pattern of seasonal fluctuation is influenced by the physical factors of light, temperature and availability of nutrient salts (mainly nitrates and phosphates). The chief biological factors involved are those produced by grazing zooplankton, including relationships between individuals of the same species (intraspecific) and between different species (interspecific).
The picture is further complicated by other factors such as turbulence, salinity, pH and dissolved oxygen. Turbulence, produced by instability of the vertical water column or by horizontally induced wind and tide movements, may act as a hindrance to phytoplankton production in transporting the organisms out of the photic zone to the dimly lit layers below, or as a help in supplying fresh nutrients for new growth. Salinity is probably not a limiting factor except in its effect on the density structure and hence vertical stability of the water. Factors like dissolved oxygen and pH are chemical manifestations of biological interactions such as the balance between photosynthesis and respiration.
In winter, the water is unstable owing to surface cooling, denser layers sinking below warmer and less dense ones. Aided by storms, the layers of water are turned over many times until the sea temperature is almost uniformly low. Weak, low-angled rays from the sun are reflected from the surface or penetrate obliquely and not far enough down to form an effective photosynthetic layer. In such an environment, where nutrients are abundant but heat and light are inadequate, no blooming occurs.
With the onset of spring, nutrients are still present after winter turbulence, and the sun increases daily in altitude, raising the surface heat of the water and illuminating deeper layers. Here, then, are the conditions under which blooming may occur, through rapid multiplication by cell division— up to six times a day or more in warm temperate latitudes (Wood 1958) —of the species which is best able to take advantage of the prevailing environmental conditions. There follows a succession of organisms, each rising to dominance, and giving way just as rapidly to others, and each possibly conditioning the presence of its successor by the liberation of certain organic substances such as hormones, antibiotics and vitamins. These substances, together with excretory and respiratory products, have been grouped together under the heading of external metabolites (Lucas 1947).
As summer advances, nutrients become scarce, since they have been used up by the spring bloom. With heating of surface layers, the water mass is more stabilised, therefore turbulence due to instability is greatly reduced. However, turbulence caused by winds will tend to produce an isothermal,
During autumn, unstable conditions again prevail with surface cooling and equinoctial gales. Once again, turbulence brings fresh phosphates and nitrates from decayed organic remains to the upper layers, in lesser quantities than in winter; but with the still ample light and heat, conditions are favourable for another bloom though on a lesser scale than that in spring.
Towards higher latitudes the season of maximum production is progressively later, until in polar seas there is only one summer peak. In the tropics, however, the seasonal effects are not pronounced, a more regular and sparser growth of many different species taking place through a much deeper photic zone at all times of the year.
A peculiar feature of seasonal phytoplankton distribution is the uncertainty of the actual species which will dominate in the succession from one year to the next. The situation is made even more complex by the difficulty of sampling an area adequately. Some species seem to bloom as a result of an inherent pulse or rhythm of a genetic nature, with no apparent relation to the prevailing nutrient and temperature cycle. Intensive culture in the laboratory of the organisms concerned will help to elucidate this problem.
Most authors recognise two main groups: —
So main terms are used in describing a diatom cell or frustule that precise definitions are required to gain a clear picture of its structure and organisation. (Plate I, Figs. 1-4.)
Basically, the structure of each diatom cell is that of a box. The lid and bottom of the box are the valves, and the sides are formed by the girdle or connecting bands which overlap slightly, one half of the cell being larger than the other. The overlapping connecting bands are known as the girdle (Plate I, Fig. 1). But this box-like cell differs from all other algal cells in being encased by a skeleton of silica which is often modified to form intricate shapes and is transversed by numerous canals and pores which connect the inner protoplasmic contents with the external aqueous environment. Recent studies under the electron microscope made at very much higher magnifications than are possible under an ordinary light microscope have shown that the siliceous wall is an amazingly complex structure. A thin cytoplasmic layer lines the pectic membrane of the wall. The nucleus may be suspended in the threads crossing the central vacuole, or may lie adpressed to the wall. Chromatophores, constant in shape and size for each species, vary greatly from small discs to large, simple or complex anastomosing plates. Some chromatophores of pennate diatoms contain one or several pyrenoids. The olive green pigment is made up of chlorophyll a and c, B-carotene, fucoxanthin, neofucoxanthin a and b, diatomin and diatoxanthin (Strain 1951). A fatty oil, the principal food reserve product of diatoms, is thought to be the main cause of ‘slicks’— glassy patches or
Average diatom size actually becomes less as a population increases in numbers. When a diatom divides by mitosis, nuclear division is usually followed by the separation of the two halves of the parent cell and cleavage of the cytoplasm into two parts. Each new protoplast is thus partly naked and partly encased by part of the parent cell wall, which becomes the new epitheca. A new wall, fitting inside the parent one, is rapidly secreted by the protoplast. This smaller wall is the new hypotheca.
Owing to the box-like organisation of the frustule, with successive divisions the descendents of one half of the box become progressively smaller until further division by ordinary vegetative means becomes impossible. Consequently a curious process of spore formation has evolved whereby on rare occasions the protoplast escapes from the firm surrounding wall and expands to about the original size of the cell. Among pennate diatoms a sexual fusion occurs (two parent cells conjugate in Navicula and Nitzschia). In centric forms like Thalassiosira, the two halves of the cell wall become separated by the enlarging protoplast, which surrounds itself with a thin, mainly pectic, membrane. Valves and girdle bands are soon formed inside this, completing the formation of the new and enlarged individual. It is now known that centric diatoms also reproduce sexually, i.e. they too are diploid, and auxospore formation is a sexual process (see summary of research on this topic in Papenfuss 1955). Thus the auxospore is a true zygote.
Other spores, called microspores, are produced much more frequently as a result of nuclear division without cell division. Up to 32 flagellated microspores are known in Biddulphia mobiliensis. Resting spores, one per cell, with thick walls and concentrated contents, may tide many diatoms over harsh periods. In Chaetoceros these are covered with spines (Plate II, Fig. 5). Under certain conditions in Ditylum resting spores may germinate to form auxospores (Gross 1940).
All that is required to collect these little plants is a fine-meshed conical tow-net of bolting silk or nylon of about 200 meshes to the inch, with either a plankton bucket or simply an open jar placed tightly in the narrow, cylindrical canvas sleeve at the base of the net. The wide end of the net needs to be attached to a metal hoop and fixed by three strong cords to a towing rope. A tow at slow speed (not more than two knots), just below the surface, for fifteen minutes should be ample for a good catch. 3% to 5% neutral formalin is adequate to preserve the plant material, but if there is an abundance of small animal larvae or copepods, up to 10% is needed.
For many of the delicate plant forms, e.g. Rhizoselenia and Lauderia, further treatment is unnecessary, even harmful, wet mounts on a clean slide covered by an 0.0 or 1.0 coverslip being quite satisfactory. For the more heavily silicified types like Coscinodiscus and most pennate forms much severer treatment must be used to eliminate the organic contents and enable the intricate cell wall structurs on which identification largely depends to be seen clearly.
Several methods for clearing diatoms have been described. Perhaps the most usual one is that of boiling the material in hydrochloric, nitric or sulphuric acid, and adding potassium chloride or sodium nitrate. However, the process is violent and must be performed in a fume cupboard. An easier method is that involving the reaction between hydrogen peroxide and potassium dichromate or potassium permanganate (Van der Werff, 1955).
The material is centrifuged and washed in distilled water several times, then placed in a porcelain dish, flooded with 30% hydrogen peroxide, and covered with a piece of glass. After standing for about five minutes, 1 mg, of powdered potassium dichromate or potassium permanganate is added carefully. After a few minutes bubbles are formed and then a more violent reaction occurs, during which the temperature rises to about 80 deg. Centigrade. The gases emitted are not dangerous, and no heating is necessary. If there is any precipitation (of hydrated manganese dioxide) after the manganese reaction, it can be dissolved by adding some 10% acetic acid. One or two rinses in distilled water should be adequate, after which the material to be examined is allowed to settle, placed in drops on a coverslip (care should be taken to avoid massing of organisms by blowing gently on the drop) and left to dry completely. A drop of mounting medium is placed on the slide, preferably one with a high refractive index, e.g. hyrax or pleurax, though Canada balsam will do. Diatoms can be mounted straight from distilled water into hyrax, but must be transferred to pleurax from 95% alcohol. The coverslip is dropped diatom-face downwards, on to the gently warmed slide, and left for several days to harden.
The writer would like to express her thanks to Mr. C. T.
This note describing an easy, quick and successful method for reducing fresh or unpreserved zoological specimens to the state of clean skeletons is published for the benefit of those interested in both wholesale and small-scale results which are not time-consuming to achieve.
On parts of the New Zealand coast where there is little wave action but where the water is freely exchanged, as in harbours, with each tide, small marine carnivores accumulate at certain seasons of the year and rapidly eat the flesh from any offal such as fish heads or bodies left in the water. At first only the softest parts are eaten (muscles, nerves, fat, gills and blood vessels), the animals crawling in beneath the skin from cut surfaces of the body; subsequently the lining of the gut and the tougher connective tissue and the orbits and skin are eaten, and finally the ligaments between bone or cartilage and remnants of skin are removed so that all that is left is a disarticulated skeleton. With little modification these circumstances can be exploited for the controlled preparation of skeletons.
The method may be applied to any sizeable fresh or unpreserved vertebrate specimen. A suitable way of ensuring that the specimen to be cleaned is not carried away by currents or large carnivores is to place it in a tethered frame covered with wire netting; the mesh size should be sufficiently small to prevent isolated parts of the skeleton such as girdle and gill arches from falling through. For some purposes, such as class material for teaching the skull foramina of the dogfish or mammal, the flesh-covered specimens can be placed as bait in an ordinary crayfish pot to be dropped below tidemark and then recovered when clean. For delicate or critical material it is probably best to use individual net or glass containers which can be recovered and opened without disturbing the arrangement of the collapsed skeleton.
The order in which the soft parts are progressively eaten varies with the specimen subjected to cleaning, but even the less accessible softer parts are usually eaten before the more accessible tougher parts such as skin, mouth lining and fibrous skeleton. With experience, the process can be allowed to continue until the desired stage is reached, and for specimens to be
The most effective scavengers are the voracious sea lice, Cirolana (Isopoda) of which there are two species identified by Dr. C. rossi, the larger one, and C. arcuata, the smaller. The rate of skeletonising by these carnivores can be phenomenally high. Fish caught in set nets are often attacked by sea lice which enter at the gills and mouth and eat their way beneath the skin back to the base of the tail, and in this way they are reduced overnight to nothing but skin and skeleton. This is a fraction of the time required for the full action of even the most vicious chemical macerating fluids.
Since the method is an ecological one, a similar ecological situation in other parts of the world will have its list of scavenger species to help in the preparation of skeletons. In South California the local sea louse used is Cirolana harfordi. In Otago Harbour, cockabullies (Tripterygion), the whelk (Buccinulum), and a number of species of small crabs have been found during skeletonisation at low-water mark, doing the final cleaning of specimens after the departure of the sea lice which work mainly at night. After the biological process has been stopped, little more cleaning may be required than can be done under a running tap with a pair of forceps to remove sediment and scraps of tissue. Further treatment by leaving skeletons in cold, fresh water for further maceration by bacteria may be tried but has not proved satisfactory for cartilaginous skeletons, and is not necessarily desirable if the action of scavengers has been really effective. Cartilaginous skeletons soon become soft and fall apart, so that if the scavenger fauna is not rich enough for them to be cleaned within a few days, leaving them longer will not be successful.
The advantages which this method of skeletonisation has over other biological methods such as bacterial decomposition, cleaning by insect larvae or ants, are: (i) it is quick; (ii) it can be observed and readily controlled without unduly disturbing the process by digging for the specimen; (iii) the skeleton is not discoloured by earth staining or faeces; (iv) the specimen is continually washed.
In angiosperms the advent of flowering is accepted as a matter of course, and, in fact, the majority of these plants do produce flowers when internal and external conditions are favourable. Occasionally, however, one may notice a plant in which one or more flowers are deformed, or are in some way different from the remainder. In many cases the nature and causes of the deformity are not known, but there is one type which has been the subject of a considerable amount of work in recent years. In this, the floral axis, instead of producing normal sepals, petals and stamens, produces structures which bear some resemblance to ordinary leaves while the apex continues to grow instead of giving rise to the gynoecium. It would appear as if the plant had started the formation of a flower and then, when it was almost too late, changed instead to vegetative growth. Such a phenomenon is known as VEGETATIVE PROLIFERATION.
This type of abnormality has been observed in many plants representing most major trends of angiosperm evolution. The present author has noted it in plants as widely different as the garden Penstemon and the rush Juncus articulatus, as well as in a flowering peach and a number of sedge species. Other reports show that proliferation has been observed in roses, lupins and the native flax Phormium tenax. However, it is in the grass family that proliferation is most frequently found and it is in this group that the subject has been most thoroughly investigated.
The ultimate unit of a grass inflorescenice is the spikelet and it is necessary to understand the structure of this unit before looking into the nature and causes of proliferation in the group. A typical spikelet (Figs. 1 and 2) is a branch of determinate growth, bearing leafy bracts or glumes. The two lowermost glumes are usually sterile while each of the upper ones bears a flower in its axil. These fertile glumes are termed the lemmas. On the floral axis is first a prophyll or palea and then two small bracts the lodicules, these latter structures representing the reduced perianth. The stamens are attached above the lodicules, while the gynoecium occupies the apex of the flower. Thus the typical spikelet is composed of a number of flowers, together with their associated bracts or glumes, borne laterally on the spikelet axis.
In grasses the spikelets rather than the individual flowers proliferate and they do so in one of two ways. Firstly, the axis or rachilla may remain
As is the case with most natural phenomena, many intermediate stages can be seen between the normal type of spikelet and either of the proliferation types, and although intergrades between the two proliferation types are not found, both of these can be observed on the same inflorescence. Despite their different appearance and structure, it is believed that both are effects of the same general cause.
When proliferation in grasses was first reported, its nature was not fully understood and the term vivipary was applied to it. This is an unfortunate use of a term which should be applied to the premature germination of seed which is still held by the parent plant, as is found in the mangrove Rhizophora. It is, however, an understandable error since the proliferating spikelet giving rise to a bulbil bears a striking resemblance to a spikelet containing germinating grains. Some authors overcome the difficulty by calling proliferation false vivipary, but this is unsatisfactory as it implies some connection with the viviparous habit. The most recent trend is towards the use of the term spikelet proliferation. However, the term vivipary will remain because races which regularly reproduce by the production of bulbils bear the varietal name vivipara, e.g. Poa alpina var. vivipara.
Spikelet proliferation has been recorded in some thirty grass species, including many of the important agricultural grasses. Some species regularly reproduce by the production of bulbils and these are referred to as the
Poa alpina var. vivipara and Deschampsia alpina of the European artic-alpine areas. In these, proliferation is an hereditary character scarcely modified by the environment. Other species only occasionally produce proliferating spikelets and they are said to show EPHEMERAL PROLIFERATION. In the northern hemisphere many grasses are reported to have shown ephemeral proliferation while in New Zealand the following species are known to have produced bulbils and there are possibly many others: cocksfoot, Dactylis glomerata; crested dogstail, Cynosurus cristatus; timothy, Phleum pratense; and a native meadow grass, Poa lindsayi. These species show a greater tendency to proliferate than others and in cocksfoot it is almost a regular feature in late autumn.
In a high proportion of the grasses which have shown a tendency for proliferation the chromosome numbers are extremely variable. Some of the species concerned have developed a greater or lesser degree of polyploidy while others have developed chromosome numbers which are not direct multiples of the haploid number for the species and these are said to be ANEUPLOIDS. As an example, the species Deschampsia alpina has chromosome numbers of 26, 39, 41, 48, 52 and 56, with a basic haploid number of 13. A feature of aneuploid species is that they tend to be sterile, rarely producing fertile seed. Nygren (1949) in a survey on the relation between chromosome number and degree of proliferation developed in Deschampsia alpina, showed that the plant with a diploid number of 26 was perfectly normal and produced flowers which set ferile seed, while plants with numbers from 39-49 showed, with increasing chromosome number, an increasing tendency to proliferate. Thus with increasing sterility induced by the greater degree of aneuploidy, there is a greater tendency for plants to produce proliferating spikelets. In these circumstances natural selection would favour those plants which can reproduce their kind vegetatively.
The viviparous races are all highly aneuploid and are probably incapable of producing fertile seed. Proliferation in these is an hereditary character determined directly by the genotype and, because they always reproduce by this means, all members are genetically identical, or in other words they form a clone.
In those grasses which show ephemeral proliferation, control is not entirely genetical, the environment playing a large part in controlling the development of bulbils. In the majority of cases the proliferation is noted in the autumn when conditions are becoming cooler, the air is frequently more moist and the length of day is becoming shorter. It was first suggested that the damp conditions prevalent in autumn induced proliferation, but it has subsequently been shown that, although damp conditions are necessary for the establishment of the bulbils and hence those species which show a tendency to proliferate are found in regions where rainfall is plentiful, these conditions are not the actual cause of the proliferation.
It is thus apparent that proliferation can be caused either by irregularity in the chromosome number of the species, or conditions unfavourable to flower production, and in most cases both of these causative factors are working together.
In a recent article, Sussex (1956) has reviewed the causes of flowering in angiosperms, and it was shown that the length of day is one of the characters of prime importance in determining whether a plant will flower or not. Brief mention was made of the necessity, in many cases, for the exposure of germinating seed or young plants to the effect of cold temperatures. Many plants which flower only when the length of day is above a certain minimum and are hence classified as long-day plants, will not flower, even when exposed to the correct photoperiod, unless they have been subjected to a period of cold temperatures. This pretreatment is known as VERNALISATION. Exposure of plants to cold temperatures during germination or early growth was first thought to be the only way of vernalisation, but then in a few species it was found that if young plants were grown in short-day conditions they would react as if they had been exposed to low temperatures. Thus in these species, vernalisation can be given in the form of low temperatures or short days and in natural winter conditions both are given together. The exposure of plants to suitable periods of vernalisation followed by adequate daylengths is believed to promote the formation of a hormone, as yet not isolated, which induces firstly the formation of the flowering branch or, in grasses, the culm and later the formation of the flowers. Although the nature of this flower-promoting hormone is not known it is conveniently called FLORIGEN.
Most grasses which show ephemeral proliferation are from temperate or cool-temperate regions where they receive a period of vernalising conditions in the winter followed by long photoperiods in the summer. Thus they can be classified as long-day plants.
In the most recent work on proliferation (Wycherley, 1954) two possibilities have been presented to account for the production of vegetative spikelets.
Firstly, that the critical concentration of florigen for culm initiation is the same as for flower initiation but, as inflorescence development proceeds, the hormone may be used up so that at the time of flower initiation the concentration of florigen is below the required minimum and the spikelets proliferate.
Secondly, that the critical concentration of florigen necessary to induce culms may be lower than that required to induce flowers and if the concentration of florigen at culm initiation is at the minimum then there will not be enough for flower initiation and the spikelets will proliferate.
The second postulation is the one most favoured. It is suggested that in plants showing ephemeral proliferation, the concentration of the flowering hormone necessary for culm and flower formation may be nearly the same and hence rarely will there be insufficient to complete the formation of flowers. However, by the end of the growing season most of the florigen will have been used up in the formation of earlier culms and inflorescences and no more will be produced because of the unfavourable photoperiod. Thus in these plants proliferation is most prevalent in the autumn when the concentration of florigen will be at a minimum for culm initiation and insufficient for flower initiation. In the viviparous races, on the other hand, the critical level of florigen for flower initiation may be so much higher than that for culm development that the plants always proliferate.
In both these races, i.e. those showing ephemeral proliferation and the viviparous races, the character of proliferation is genetically controlled, in that the genotype determines at what concentration of florigen the processes of culm and flower initiation will take place. In the former it is indirect control, as the concentrations of florigen necessary for culm and flower initiation are almost the same, the environment controlling the level of florigen in the plant, while in the latter it is direct control.
These explanations are purely hypothetical but until the nature of the flowering stimulus is more fully known little further progress can be made. From these observations on proliferation it would appear, however, that flowering is not an all-or-nothing phenomenon as has been postulated previously.
Thanks are expressed to Professor
Starfishes and brittlestars are anatomically very similar and were formerly grouped together in a single class, the Stelleroidea, which included all star-shaped echinoderms. This simple classification, which was still in use in 1900 (when Gregory employed it in Lankester's Treatise of Zoology) was subsequently displaced by another, in which the starfishes (Asteroidea) and brittlestars (Ophiuroidea) were elevated to rank as distinct classes. The change was introduced on the grounds that embryological investigations suggested that asteroids were more closely related to holothuroids, since their larvae were similar, whereas ophiuroids had larvae which resembled those of echinoids. This view, although still maintained by Hyman in 1955. is no longer acceptable to many students of echinoderms. Fossil studies have shown that it is extremely unlikely that asteroids are more closely related to holothurians than to ophiuroids, or that ophiuroids could be more closely related to any known echinoids than they are to asteroids. Indeed, it can hardly be disputed that both asteroids and ophiuroids descend from common Palaeozoic ancestors which are now known to us -- namely, those star-shaped forms called Somasteroidea. Thus the larval evidence must have been misinterpreted, and requires reassessment.
After a lifetime of more than fifty years devoted to the study of fossil echinoderms, W. K. Spencer (Phil, Trans., 1951) set out in his last paper a new classification of the star-shaped echinoderms; in it, he revives the older grouping Stelleroidea, ranking it as a single class, and including in it three subclasses, two of them corresponding to the familiar asteroid and ophiuroid groupings, the third one comprising the Somasteroidea. This treatment has been adopted in the Anglo-American Treatise of Paleontology, now in preparation by a group of specialists, and probably will remain standard for some time to come. It may be noted that these authors prefer to use the name Asterozoa instead of Stelleroidea, parallelling another assemblage of echinoderms termed the Echinozoa. Diagnoses of the three groups of Asterozoa are too technical for presentation here but the following notes may serve to illustrate their character:
Class: ASTEROZOA— Free-living echinoderms with a depressed, stellate body; the ambulacral skeletal plates lie internally with respect to the radial ambulacral vessels; the madreporite is externally visible, but is not incorporated into any system of apical plates.
The foregoing paragraphs have referred to the general systematic position of starfishes. The following are some notes on the habits and ecology of starfishes, and their relation to other animals.
Starfishes usually creep about the sea-bed with the aid of their tube-feet. No swimming forms have yet been recorded. Those species which have suctorial tube-feet are able to climb and descend obstacles with their aid, and capture food. A number of genera have peculiar peg-like tube-feet (Fig. 2A), without suckers; they usually occur on submarine mud-banks, where they seem to use their tube-feet as oars or as stilts perhaps— I imagine them as tottering or paddling through the semi-fluid, slimy mud.
Although starfishes cannot see, since they have no eyes, they are able to detect light-fluctuations with the aid of a photosensitive eye-spot at the outer end of the ambulacral groove where it reaches the tip of the arm. Starfishes cannot be said to ‘ hear ‘, though they evidently detect the grosser vibrations often associated with sound. At the tip of the arm are some special tube-feet in some starfishes, which are held erect when the animal is active, and probably serve as taste-organs (chemoreceptors).
The sex of a starfish is seldom obvious without dissection, but in those forms which carry or brood the young, the female can be recognised during the breeding season (see Fig. 42). Their breeding habits are very varied. Apart from brood-protecting forms such as Calvasterias in New Zealand littoral waters, there are deep-sea forms such as Pteraster (Fig 28) with a dorsal brood-chamber developed on the upper side, provided with a water-conditioning system not unlike the water-circulatory system of a sponge, and having a large central osculum for the outflow of the water-current. One Greenland genus, Leptychaster, somehow contrives to hatch its eggs inside its stomach without accidentally digesting them. Many starfishes have free-swimming larval stages, others lay yolky eggs which undergo a direct development.
Starfishes are probably short-lived animals with a life-span of only a few years. Some, at least, are known to be sexually mature after one year, though growth continues for about four years. Some kinds can regenerate lost arms; Luidia (Fig. 7) fragments upon even gentle handling, and must spend much of its life regenerating its lost members, to judge by the unevenly matched arms its species often exhibit. In New Zealand waters another genus. Allostichaster (Figs. 38-40) regularly reproduces by asexual transverse fission. In the tropics Linckia can regenerate the whole animal from a single arm. producing curious ‘ sea-comets ‘ in the process; in Cook Strait a species of Sclerasterias sometimes shows the same habit.
The brilliant colours of starfishes are due to biochromes which usually fade on preservation, and are chemically changed by alcohol. The colours include orange or vermilion (the two commonest), crimson (deep-sea forms), yellow, green, blue, violet, brown, as well as variegated patterns.
There are usually five arms, but many species have more than five, extreme cases occurring among the deep-sea Brisingidae, where as many as 44 long fragile arms may be present. Square starfishes with only four arms are abnormalities, but by no means rare. The largest starfishes (Linckia spp.) reach a metre in diameter; the biggest New Zealand species, Astrostole scabra (Fig. 41), is about half this size.
Starfishes are fast-growing animals and require a great deal of food. The carnivorous types will attack any animal which they can swallow whole, and will feed upon pieces of larger animals if the stomach can be applied to the food. Some species with suctorial tube-feet attack bivalve shellfish and sea-urchins. The valves of shellfish are forced apart by long-sustained suction exerted by the tube-feet and the arms; in this way members of the family Asteriidae do much damage to oyster fisheries.
Starfishes are not known to be dangerous to man, though tropical species occasionally cause septic wounds in the foot, as in the case of the Fijian genus Acanthaster, a coral-reef dweller with sharp spines. No New Zealand species is likely to cause injuries of any kind. Starfishes are not known to be poisonous to eat, though they are said to be bitter and unattractive. Of the few animals which do use them as food the walrus is perhaps most notable.
A number of parasites attack starfishes. A sea-snail, Stylifer, bores into their skin; a polychaete worm, Achloe, inhabits the ambulacral groove, a cirripede destroys the sex-gland of Coscinasterias and others, and in the tropics a slender fish, Fierasfer, inhabits the body cavity (apparently entering the mouth of its host, and boring through the stomach wall).
There are about 1,500 known species of starfishes, inhabiting the coasts of all known seas, but avoiding brackish and fresh water. Some live at low-tide level, or float on sea-weed at the surface of the sea (Calvasterias. Fig. 42); others burrow into the sea-floor. Twelve families are known to range into waters more than two miles deep, and one species (Porcellanaster caeruleus) has been recovered from the sea-bed at a depth of four miles. Next to holothurians, starfishes are the deepest-dwelling animals of the sea —
probably because, like holothurians, they can extract nutriment from mud. Individual species usually have rather restricted bathymetric ranges, but where the continental slope is relatively steep, as it is off New Zealand, deep-water forms often ascend the slope, and shelf forms often tumble into deep water; thus the content of Cook Strait hauls is rather unpredictable.
From New Zealand waters 35 genera, embracing 46 species, are now known; six families have been added in the last few years, and one other family. the Porcellanasteridae. will probably be discovered here soon (it has been included in the key). It would now appear that there is a larger Indonesian and Australian element in the fauna than was formerly supposed, especially in the northern half of New Zealand. The supposed South American affinites seem to be negligible, and are perhaps a result of the epiplanktonic drift of seaweed-inhabiting forms, or the dispersal of planktonic larvae. Asterodon, a genus well represented in New Zealand waters, may have contributed a species to South America by way of the west-wind drift and Humboldt current. Certainly the main affinities of the New Zealand asteroid fauna are with the Indo-West-Pacfic, not with South America. A few deep-water species resemble North Atlantic forms; perhaps they are really cosmopolitan.
To use the key which follows, take each numbered paragraph consecutively. so long as the characters of the specimen in hand agree; if a character differs, go directly to the number shown in brackets. The key is complete to date, and is substantially systematic, so that the taxons occur in their accepted natural order. The length of the arm is given in the form of the major radius (R), measured from the centre of the disc to the tip of the arm, and is for an adult specimen. The colour in life is given, when known, and omitted if only preserved material has been described. The series of illustrations is representative, though not complete. In many cases it should be possible to make an approximate determination merely by inspection of the illustrations; the identification should then be checked against the key. Since many species undoubtedly remain to be discovered, a final determination can only be made against a complete description of each species. Unfortunately the literature is too scattered for citation here.