200 INVERTEBRATE MORPHOLOGY.. 



in the substance of the supraoesophageal ganglion, and tactile 

 hairs occur at various regions of the body. 



Affinities of Dinophilus. The descriptions given of the various known 

 species of Dinophilus indicate a considerable variation in the structure of 

 certain parts, more especially of the nervous system, which in D. tceniatus 

 partakes of the metamerism shown by the nephridia and the bands of cilia, 

 while in other forms it is apparently non-metameric. This would indicate 

 either that the metamerism has been acquired within the limits of the 

 genus, or else that those forms lacking it are degraded in this respect and 

 have descended from metanieric ancestors. There is little justification to 

 be found, however, for the calling in of degradation to explain obscure re- 

 lationships unless there is sufficient collateral evidence to support such an 

 appeal ; in the present case this seems to be absent, and the marked simi- 

 larity of the non-metameric nervous system to that of the Turbellaria sug- 

 gests an origin from these forms and favors the first hypothesis as to the 

 origin of the metamerism. The nephridia also and the character of the 

 ccelom strengthen the probability of a Turbellarian ancestry. 



A close relationship to the Rotifera has also been suggested and is not 

 debarred by the supposition of a descent from Turbellarian forms ; but it 

 seems doubtful if such a relationship can be other than a very distant one. 

 The position of the supraoesophageal ganglion relatively to the cephalic 

 cilia or prototroch, and the paired arrangement of the nephridia as well as 

 the occurrence of circular fibres in the subepidermal musculature, stand 

 in opposition to the view, and the most that can be said is that both 

 Dinophilus and the Rotifera are to be referred back to closely-similar 

 ancestors. 



The affinities of Dinophilus and the Rotifers to the Annelida will be 

 discussed in connection with the latter group (p. 217). 



SUBKINGDOM METAZOA. 



Order Echinodera. Body cylindrical, with 11 rings ; no cilia ; with pro- 

 boscis ; minute forms ; marine. Echinoderes. 



Class CH^TOGNATHA. Marine ; body divided into three segments ; with 

 lateral and tail fins ; mouth with chitinous jaws composed of 

 series of strong bristles. Sagitta, Spadella. 



Class ROTIFERA. Anterior end provided with a retractile crown of cilia ; 

 minute forms both aquatic and marine. Flosctilaria, Melicerta, 

 Lacinularia, Philodina, Brachionus, Asplanchna, Trocho- 

 sphcera, Pedalion, Hexarthra. 



Order Gastrotricha. Minute forms both marine and aquatic ; ventral sur- 

 face of body ciliated ; no anterior crown of cilia. Ichthydium, 

 Chwtonotus. 



Genus Dinophilus. Small marine forms ; body with 5-7 segments, each 

 with a ring of cilia. 



