132 



EVOLUTION AND GENETICS 



The Mollusca. This is the highest group of unsegmented 

 invertebrates that has yet appeared. They are characterized 



by (1) the absence 

 of appendages, (2) a 

 ventral ' ' foot ' ' which 

 is usually associated 

 with locomotion, (3) 

 the mantle, a dorsal 

 fold of the body 

 wall which often 

 secretes a shell and 

 encloses a space in 

 which lie the gills, 

 (4) a heart consisting 

 of two auricles and 

 a ventricle, (5) a 

 tubular vascular 

 system associated 

 with spaces called 

 lacunae in which the 

 blood also circulates, 

 and (6) develop- 

 mental stages of 

 which one resembles 

 some larval annelids. 

 The Cephalopoda 

 are apparently the 

 most highly devel- 

 oped class. They 

 include the four 

 existing species of 

 Nautilus, the cuttlefishes, squids, Octopus, Argo7iauta, etc., all 

 of which have the highly developed eye described in a pre- 

 vious chapter. 



The shell is, of course, the part of a molluscan body most 

 easily preserved as a fossil. Consequently it is on the change 

 of structure of the shell that our knowledge of phylogeny is 

 based. The phylogeny of the group is indicated in the follow- 

 ing diagram, which has been modified to indicate the change of 

 shell structure. 



Fig. 71. — A crinoid, Pentacrinus madearanus. 

 (From the Cambridge Natural History.) 



