TYPE MOLLUSGA. 293 



The structural peculiarities of the Polyplacophora point strongly to 

 their primitive character, though iu many respects they are less primitive 

 than the Solenogastres. Tlius they possess special reprod uctive ducts, in all 

 probability a secondary acquisition, and furthermore the reproductive and 

 pericardial moieties of the enteroccel no longer communicate freely. If 

 the Solenogastres have been derived from forms with Chiton-like shells 

 (see p. 388), then it must be supposed that the tvpo groups represent di- 

 verging lines of development from a common ancestor whose character- 

 istics have been partly retained in the one group and partly in the other. 



II. Class Gasteropoda. 



The Gasteropods form a very complex group, the various 

 members differing so much in the details of their organiza- 

 tion that it is difficult to give a general description which 

 will apply to all the forms. Certain features may, how- 

 ever, be considered typical of the class, and these may be 

 mentioned here, reserving notice of the more important varia- 

 tions until the various subdivisions are being considered. 



One of the most characteristic features is the occurrence of 

 what may be termed the " visceral hump " whose presence is 

 responsible for many of the peculiarities of Gasteropod struc- 

 ture. It consists of an elevation into a dome-like structure 

 of the dorsal region of the body, the digestive tract and 

 gland being contained within the elevation. The mantle 

 arises as a circular fold surrounding the hump, but usually is 

 more highly developed, and therefore encloses a deeper 

 cavity, upon the right side or anterior surface of the hump, 

 and in the cavity so arranged lie the structures which usually 

 are associated with the mantle-cavity, namely, the branchiae 

 and the openings of the digestive tube and of the nephridia. 

 There is thus a very decided asymmetry in most Gasteropods, 

 usually emphasized by the visceral hump being coiled into a 

 spiral, a coiling which is shared by the shell, usually present 

 and consisting of a single tubular structure surrounding the 

 visceral hump, but usually sufficiently ample to permit of the 

 retraction within it of the rest of the body. 



In a number of forms the visceral hump may be very 

 much reduced, and with this reduction there is generally con- 

 comitant a reduction of the shell, but such conditions are 



