ABERRANT ANIMALS-THE MOLLUSKS AND ECHINODERMS 253 



ternally to be segmented, show no true 

 metamerism when the internal anatomy is 

 studied. It seems rather strange that this 

 group of animals ignored or failed to ac- 

 quire a body arrangement which is so suc- 

 cessful among the annelids and arthropods 

 and becomes even more so anions the 

 chordates. 



In considering the ancestors of the mol- 

 lusks it is necessary to go back a very long 

 way, into prehistoric times. Fossil records 

 indicate that mollusks were present in some 

 of the earliest rocks and have continued, 

 uninterrupted, to the present time. How- 

 ever, even in the rocks there is little evi- 

 dence to determine their ancestry. It is 

 generally believed that they were derived 

 from "worm-like" ancestors, although these 

 were not the annelids known today. Larval 

 studies indicate that there is a close rela- 

 tionship to the annelids, but that relation- 

 ship must go far back since the larval stage 

 of the mollusk, the trochophore, does not 

 show segmentation in the mesoderm (Fig. 



12-2). This means that the annelids and 

 mollusks split off and went their separate 

 ways before segmentation was introduced. 

 The trochophore larva is common to both 



Fig. 12-1. Chitons are primitive mollusks and probably 

 resemble the ancient forms that gave rise to our 

 modern mollusks. Note the eight overlapping shells 

 on the dorsal side. Chitons live among the rocks on the 

 seashore and are active at night. 



trochophore 

 larvae 



mollusca 



annelida 



Fig. 12-2. The ancestral trochophore larva from which 

 both the mollusca and annelida are thought to have 

 been derived. 



annelid and mollusk and remarkably simi- 

 lar in both. The coelom is formed by a de- 

 pression in the mass of mesoderm, which 

 arises from a single cell. 



The molluscan body plan has certain 

 characteristic featm-es that appear con- 

 sistently in all of the species in the group. 

 One of these is a muscular organ, the foot, 

 an organ which serves for several tvpes of 

 locomotion. The snail and chiton crawl on it, 

 the clam digs a wedge-shaped path with 

 it and also walks on it, while the squid uses 

 it to capture prey as well as to crawl over 

 the ocean floor (Fig. 12-3). Another new 

 character is the mantle, which is an enve- 

 lope of tissue covering the entire animal. 

 The mantle gives rise to the shell, common 

 in so many members of this group. The 

 original shell appears in the larva as a 

 product of the mantle epithelium and 

 gradually expands as the animal grows. 



Those mollusks that possess shells use 

 them as an abode which is readily available 



