35 



muscles.* From this ancestral Cephalopborous or Glosso- 

 phorous Mollusc, which is continued up into the primitive 

 Gastropoda, two lines have diverged. The first, with compa- 

 ratively little modification, to the Isopleura, and the second, 

 with a considerable amount of divergence, to the Scaphopoda. 



In the Isopleura, the primitive bilateral symmetry has 

 been retained, and most of the systems remain very much in 

 the condition in which they were found in the Archi- 

 Mollusc, with, of course, the addition of the odontophore. 

 The Isopleura include three groups : — the Polyplacophora, 

 or Chitons, in which the simple dorsal shell has been 

 multiplied so as to form a series of eight valves ; and the 

 Neomenise and Chaetoderma in which the shell is repre- 

 sented by numerous minute calcareous plates or spines, and 

 the mantle and foot are much reduced, and the body is 

 worm-like in form. These are the most primitive Gastro- 

 podous Molluscs which are known. 



The Scaphopoda {Dentalium) are much more modified, 

 and possibly somewhat degenerate. They have retained the 

 primitive bilateral symmetry, but the body has become 

 greatly elongated antero-posteriorly, and the foot is especi- 

 ally produced anteriorly, and adapted for burrowing in sand. 

 The mantle-skirts have fused ventrally below the foot, so as 

 to produce a cylindrical body-form, and around this the shell 

 is developed as a cylindrical tube, open at both ends. The 

 heart seems to have been lost during the stages which have 

 intervened since the Scaphopoda diverged from the primitive 

 Gastropods. 



Eeturning to the line leading upwards to the higher 

 Cephalopborous Molluscs, we find that after the ancestral 

 Scaphopoda had diverged, the visceral dome must have 

 increased greatly in size, so as to form a great dorsal pro- 



* For a description, with figures, of the odontophore, see Lankester's 

 article, '* Mollusca," Ency. Brit., 9th edition, p. 640. 



