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Studies on Gyrocotyle rugosa Diesing, 1850, A Cestodarian Parasite of the Elephant Fish, Callorhynchus milii

Description of the lycophore larva of G. rugosa (Figs. 5–8)

Description of the lycophore larva of G. rugosa (Figs. 5–8)

Extended specimens, measured in temporary mounts in 70 per cent. alcohol, were 130 to 140 by 50 to 60 microns. The body, tapering from near the middle toward each end, is rounded anteriorly and somewhat truncated at the posterior end. Free swimming larvae as they rotate are seen to fee slightly bent or indented near the middle of one surface as noted in the case of G. fimbriata by Lynch (1945). The body is completely covered by long cilia of approximately equal length. No evidence of cuticular plates could be seen. Circular muscles lie just beneath the ciliated layer. Except for the posterior hooks, in general appearance and behaviour, this larva certainly resembles a miracidium.

Almost filling the anterior third of the body are three pairs of elongated, pyri-form, transparent sacs considered to be glands. The middle pair is almost twice as long as the others and in contracted specimens may be somewhat bent or folded. These structures do not stain but appear as transparent vesicles. The underlying short pair is usually not evident from a dorsal or a ventral view. Each gland opens separately at the anterior end of the body. There is evidence that one of the shorter pairs (the dorsal?) is physiologically different from the others. Among numerous larvae which had penetrated a mass of mucus from the spiral valve of the host, a considerable number, after being stained, showed this pair darkly stained.

Immediately posterior to the glands and just anterior to the midbody lies a large conspicuous bilobed mass of cells. It extends anteriorly lateral to the longer gland cells almost to the base of the shorter glands and posteriorly approximately to the midbody. Near one surface, considered as ventral. the organ narrows to form an isthmus-like band across the body. On each side the cells extend upward and page 5 near the other surface (dorsal) extend medianly but do not meet. This organ is probably a nervous centre. Radiating from the anterior end of the body are six narrow bands of slender nuclei or cells. A central non-cellular cord or tube extends backward between the large gland cells and appears to fork immediately anterior to the isthmus of the cellular mass. Although not granular it is more refractive than the glands. Two finely granular, inconspicuous spherical masses lie side by side immediately dorsal to the isthmus. They are probably not connected with the central cord or tube since they lie at a different level.

Posterior to the bilobed mass, nuclei are more scattered but a central collection and one or two small pairs of cell clusters can usually be observed. At least one pair of flame cells is present. These were not seen in entire specimens either living or stained but were clearly observed in an active state in a crushed, recently hatched specimen. They were located in the posterior half of the body. Each was-pointed diagonally backward and inward. Except for the beginning of the excretory tubule near the flame cell, excretory tubules or pores could not be seen. This record of flame cells is the first for lycophore larvae.

The most conspicuous feature of the lycophore larva is the presence of 10 large hooks withdrawn into the posterior end of the body. In G. rugosa these hooks are unusually large, measuring 34 to 36 microns in length or approximately 26 per cent. of body length. Eversion of the hooks was not observed. Several specimens possessing eight rather than ten hooks were observed. When the hooks are retracted an appearance of eight might result if certain hooks lie directly over others. Fig. 7 shows eight hooks as seen in a crushed specimen.

The lycophore of G. rugosa differs in several respects from the lycophores of G. urna and G. fimbriata. That of G. urna has ten pairs of unicellular glands extending nearly to the midbody, and relatively shorter hooks. The lycophore of G. fimbriata has hooks 19 to 23 microns long and only about l/16th body length. It has at least one pair of very long gland cells (Lynch, 1945). The anterior pair of clear vesicles mentioned by Lynch may correspond to what I have considered gland cells in the lycophore of G. rugose.