254 GENERAL CONSIDERATIONS. 



which with paired ducts is doubtless the counterpart of the ventral set of other 

 species. The possible homologue of the dorsal gland here exists as a diffuse 

 band encirchng the fore gut in the neighborhood of the brain. The third set 

 is attached to the fore gut between the radula and mid-gut, and in appearance 

 and staining qualities differs from the other two. It is altogether possible 

 that this posterior set is of more recent, independent origin, and it is also pos- 

 sible that the diffuse glands existing in various species of Strophomenia may- 

 have had a similar origin. But at the same time it should be kept in mind that 

 these pharyngeal glands in Strophomenia lie in front of the radula and ventral 

 sahvary glands and accordingly may represent a diffuse dorsal gland. 



Since a radula exists in Limijossor talpoideus and Halomenia gravida, for 

 example, with odontoblasts and basement membrane typically located, and the 

 entire organ holding essentially the same position with reference to the ventral 

 salivary glands and the pharynx generally as in the Chitons, it is difficult to 

 avoid the belief that it was present before the Solenogastres became an inde- 

 pendent group. The radula may indeed have originated as a cuticular product 

 of the fore gut with separate teeth or as minute projections of a more or less 

 extensive buccal lining, but that this has been its history since the Solenogastres 

 branched off from the parent stock is highly improbable. It is true that the 

 radula in present day species is a highly variable structure — distichous, poly- 

 stichous, with or without a basement membrane, reduced to a corneal peg, or 

 absent altogether — but in my opinion the Limifossor and Halomenia types 

 of radulae have preserved their ancestral characters, while the others represent 

 different stages of degeneration. This is wholly aside from the discussion as 

 to which is the more primitive, the polystichous or distichous plan, a matter it 

 appears to me which cannot be settled considering the small amount of com- 

 parative anatomical data we now possess. 



As to the mid-gut there are wide variations and here again it is difficult to 

 follow the ancestral history. Where the digestive gland is not clearly differ- 

 entiated or the stomach or intestine sharply defined we certainly have the least 

 compUcated state of affairs and it appears to me to be the more primitive. The 

 Chaetodermatina are therefore more highly modified in this respect than are 

 the Neomeniina. 



In this connection the so-called anal, cloacal or branchial chamber may 

 be considered to be a development of the anus, as certain authors maintain, 

 and nowise the homologiie of the mantle cavity. The lamellae on its walls 

 in the Neomeniina are therefore modified anal folds and according to Thiele 



