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Show 48 ON OLASSI]'IOATION. (Fig. 21 ), the water-vasc~lar system. beco1nes divi.d~d into two distinct portions, one with contractile. ~nd non-mhated walls, the other with non-contractile and ciliated walls. In some Fig. 21. Fig 2l.-A. Water-vascular sy t('m of Aspidoga~ter conchicula; a: terminal pore: b, lateral contractile vessels; c, lateral eiliated trunks, that of the left siJe shaded; d, dilatation of this trunk; B, one of the larger, and C, one of tlw smaller, ciliated vessels. Turbellaria the apertures of the water-vascular apparatus are multiple; while it would seem that in others, as the Nemertidm, the apparatus becomes shut externally in the adult state, and consists mainly, if not exclusively, of contractile vessels. ~rhe difficulties of observation are here, however, very great, and I would be understood to express this opinion with all due caution. In none of these animals has any other set of vessels than those which appertain to the water-vascular system (if I am right in my view of the vessels of the Nemertidt:e) been observed, nor has any trace of a true heart been noticed. The nervous system consists of one, or two, elosely o.pproximated ganglia. This sum of common characters appears to n1e to demand the union of the Rotijera, Tv.rbellaria, Trematoda, and Tt:eniada into one great assemblage. Ought the Nematoid worms to be grouped with them ? If the system of canals, in some ca.ses contrar.tile, which open externally near the anterior part of the body (Fig. 22), and were originally observed by Von Siebold, and since by myself and others, are to be regarded as homologous with the water-vessels of the Trematoda, this ques- 1'HE SOOLEOIDA. 4-9 tion must, I think, be answered in the affirmative. It is almost the only system of organs in the Nematoidea which gives us a definite zoological crit.erion, the condition of the nervous system in these animals being still, not"vithstanding the many inquiries which ]lave been made into the su~ject,~ a matter of great doubt. Fig. 22. Fig. 22.-0xyuris.-a. Mouth. b. Pharynx. c. Commencement of intestine, and d its termination ; the intermediate portion is not figured. e. Genital aperture. f. Opening of vessels. g. Their receptacle. h. One of the vessels. z·. Cellular matter enveloping them. A portion of oue of the contractile vessels is represented above, more highly magnified. In habit and feature, the Gordiacea, filiform parasites which inhabit the bodies of insects, and leave their hosts only to breed, resemble the Nematoidea so 1nuch that I can bardly doubt that their Rystematic place must be close to that of the latter ; but positive evidence is almost wanting on this head, the extant accounts of the minute anaton1y of these animals not having received that kind of confirmation which is desirable. The struetnre of the Acanthocephala, comprising the formidable Echinorhynchus (Fig. 23) and its allies, is, on the other hand, pretty clearly made out. They are vermiform parasites, like the Tt:eniada, devoid of any mouth or alimentary canal, but provided with a proboscis armed with recurved hooks. 'fhe proboscis is supported within by a sort of rod-like handle, whence a cord is continued, to which the reproductive organs are attached. A single ganglion is seated in the "handle" of th.e probos~is. Immediately beneath the integument lies a Rone.c; of retwulated canals. containing a clear fluid, and it E |