TWO PAEASITIC FORMS OF THE FAMILT TETRAEHYNCHID^.. 339 



in form the young segments of tapeworms generally than that of 

 the former parasite. In the structure of the segment we get the 

 chitinous skin, subjacent granular layer, body parenchyma, visce- 

 ral space, and water-vascular canals. The muscular layers of the 

 parenchyma are transverse, circular, and longitudinal ; and, as will 

 be seen in fig. 21, the transverse fibres uniformly radiate between 

 the skin and visceral boundary instead of forming a meshwork. 

 The inorganic nodules are few, but similarly arranged and similar 

 in composition and structure. Within the visceral space is the 

 longitudinal water-vascular canal ; and, as seen in fig. 20, this sys- 

 tem is the counterpart of that in the former parasite. In the upper 

 segments the granular visceral material is, as usual, with no trace 

 of differentiation ; but in the lowermost segments present in these 

 parasites (about the 200th) there is a distinct separation of it into 

 spherical masses, apparently ovarian vesicles (fig. 21, d), as far 

 as can be traced, the process of development of the early zooids 

 closely approximating that detailed as observed in the Tcenia me- 

 diocanellata *. 



Bemarhs. — These parasitic forms (Tetrarhynchidae) are limited 

 to water-residents, the larvae being developed from ova taken in 

 by certain among them, and the mature creature reached in the 

 bodies of the predaceous species of fishes, mainly or wholly, which 

 make the larval hosts their food, the cycle of changes being 

 similar to that observed in the allied family of tapeworms. The 

 strong resemblance of the colonies of the parasites of the Shark 

 to that of Tcenice is evident, the one, as in the other, being a series 

 of semi-independent hermaphrodite zooids without alimentary 

 canal, and with a water-vascular system closely connected in all. 

 It is at the head end that the mature Tetrarhynchs mainly di- 

 verge from the Tcenice, though here there are the same elements, 

 only in a modified form, the limited rows of booklets on a rostellum 

 in the Tcenice being developed in the Tetrarhynch into an armed 

 lengthened proboscis, while the four or bipartite two suckers of 

 the latter family are apparently similar in structure and function 

 to those of the tapeworm. By the proboscis these parasites of 

 the Shark are linked to the tape-worms armed with booklets, 

 while by the suckers the larger form shows its relation to the 

 Bothriocephalus family. It is interesting to note the absence of 

 suckers in the smaller form, although placed under the same con- 

 ditions for maturity as the larger, inasmuch as in the other cestode 



* Qu.arterly Mieroeoopioal .Tournal, .January 187.T. p. 16. 



