198 



INVERTEBRATE MORPHOLOGY. 



are to be considered as representing the ancestral form from which both 

 they and the Rotifera have descended more nearly than the latter group, 

 or whether they are modiflcations of the Eotifer type of structure and have 

 had for their ancestors forms which were Kotifer-like in structure, it is 

 difficult to say ; though the balance of evidence seems to tip in favor of 

 the former view. 



Attention should be called, however, to a possible affiliation of the Qas- 

 trotridha with the Echinodera. If, as has been suggested (p. 186), the seg- 

 mentation of the latter has no phylogenetic significance, it is not difficult 

 to trace similarities of structure in the two groups, the principal differ- 

 ences being connected with external parts. It is by no means improbable 

 that the Gastrotricha, Rotifera, and Echinodera form a series, each of the 

 groups being of equivalent rank, and related to each other somewhat as 

 are the Turbellaria, Trematoda, and Oestoda. 



Genus Dinophilus. 



The genus Dinophilus includes some small marine organ- 

 isms all of which are referable to a small number of species. 

 The body (Fig. 96) is cylindrical and consists of a head segment 

 followed by from 5-7 trunk segments (the 

 number varying according to the species), 

 each of which bears a ring of cilia, inter- 

 rupted ventrally by a uniform cilia tion which 

 covers the entire ventral surface. The 

 head is likewise provided with a ring of 

 cilia which is usually double, one of the 

 constituent bands passing in front of the 

 mouth and the other behind it, the area in- 

 tervening between these two bands being, 

 in one species at least, occupied by smaller 

 cilia. The musculature of the body-wall 

 is but weakly developed, though both an 

 external layer of circular fibres and an in- 

 ternal one of longitudinal fibres may be 

 found, both layers being absent in one 

 species in the dorsal region. The coelom 

 is traversed by a network of branching cells 

 there being no special peritoneal layer, and 

 no musculature in the walls of the intestine. 

 The mouth is situated on the ventral surface at the junc- 

 tion of the head and first trunk segments, and leads into a 



(after 



Fig. 96.— 



gyrociliatus 



Mayeb'). 

 ne = nepbridium. 

 ov = ovary. 

 sg = salivary gland. 



