586 THE ANIMAL KINGDOM. 



highly modified Chaetopoda, but it is by no means certain that the three 

 families of Gcphyrea should be retained within the limits of a single class. 

 The affinities of (5) the Hirudinea are doubtful : they possess in develop- 

 ment mesoblastic somites similar to those of Chaetopoda, as well as pro- 

 visional renal organs. Their vascular and reproductive systems are peculiar. 

 The nephridial rete found in certain genera is perhaps a primitive feature. 

 Classes 2-5 are sometimes grouped together as Annelida. The Rotifera 

 (6) are an isolated class ; they represent in all probability a highly special- 

 ised form of Trochosphere, and retain a primitive type of nephridia. The 

 Nemertea, Trematoda, Cestoda, and Turbellaria are sometimes grouped 

 together as Platyhelminthes. The Nemertea (7) are a very distinct class. 

 The possession of an anus, of a system of vessel-like coelomic spaces, the 

 characters of their nervous, nephridial, and reproductive systems, distinguish 

 them from the other three classes named. The Trematoda (8) and Ces- 

 toda (9) are modified by parasitism, but in many respects, e.g. in the 

 character of the nervous, nephridial, and reproductive organs, resemble 

 (10) the Turbellaria. It is uncertain how far the last named class is to be 

 regarded as degenerate, or the reverse. The larva of some Polyclads is 

 beyond doubt primitive. The Chaetognatha (u), Nematoda (12), and 

 Acanthocephala (13), have sometimes been grouped together as Nematel- 

 minthes. They are, however, perfectly distinct, not only one from the 

 other, but as far as can be judged from all other classes of Vermes, and 

 the last mentioned of the three is most profoundly modified by parasitism. 



For a discussion on larval forms, see Balfour, Comp. Embryology, ii. p. 297 et 

 seqq. ; for the development of a typical Trochosphere larva, Hatschek ' On Eupo- 

 matus uncinatusj Arb. Zool. Inst. Wien, vi. (i), 1885 ; and for points of interest 

 (origin of oesophageal commissure, mesoblast of head, circular nerves of ciliated 

 rings), Id. ibid. ' On head of Polygordius? 



There are several interesting Vermian genera or groups, which do not fall 

 under any of the thirteen above-mentioned classes, and which can only be briefly 

 touched upon here. They are as follows : 



i. The genus Dinophilus^ with several species, all minute in size, and marine 

 with the exception of D. sphaerocephalus from brackish water. There is a head 

 (= prostomium), body, and short ventral tail. The head carries two eye-specks 

 and sensory hairs. It is ciliated either uniformly or in two bands. The body is 

 uniformly ciliated (D. vorticoides, D. metameroides\ or the cilia are disposed in 

 rings corresponding to the division of the body into six or seven segments, except 

 on the ventral surface, which is uniformly ciliated. The segmentation of the body 

 is superficial. A nervous system is represented by a ganglion in the prostomium, 

 giving off two anterior and two posterior nerves (JD. apatris\ or two lateral cords 

 situated in the hypodermis, which disappear in the last segment (D. gigas]. The 

 mouth is ventral, and in the first segment of the body (D. gigas) ; it leads into a 

 pharynx, oesophagus, stomach, and intestine, all ciliated. The anus is dorsal to 

 the tail. Opening just behind and into the mouth is a cavity, which contains 



