GENERAL ZOOLOGY 



modifications in adjusting to a variety of habitats; the visceral mass has 

 become coiled and rotated, and sometimes secondarily uncoiled and un- 

 wound, but for the most part the gastropods have retained the flattened foot 

 adapted to creeping. The cephalopods have evolved in adaptation to an 

 active, free-swimming, predaceous existence. Yet in all of these highly modi- 

 fied and generally successful types, we can still trace the basic plan of 

 organization of the ancient ancestral mollusk. 



One of the peculiar and somewhat puzzling features of the moUuscan body 

 plan is the absence of segmentation at any stage. This is the more striking 

 when moUusks are compared in this respect with members of the phylum 

 Annelida, to which we shall devote our attention in the next chapter. 

 Annelids and mollusks have obviously descended from a common ancestry; 

 their cleavage patterns, larval types, general level of organization, and other 

 features are very similar. Yet moUuscan larvae transform into the adult 

 condition without showing a trace of segmentation, whereas the larvae of 

 annelids begin very early to develop the serial repetitions of parts characteristic 

 of the phylum. 



A rather startling recent report may be interpreted as indicating that 

 mollusks were not always unsegmented, and perhaps as revealing a stage in 

 the evolution of the body plan of modern forms. A new species of limpet- 

 like mollusk has been dredged from the deep Pacific waters oflf the coast of 

 Mexico; externally it presents many interesting features and resembles most 

 closely a group of mollusks previously known only from fossilized shells dating 

 from the Cambrian and somewhat later. This new species, Neopilina gala- 

 theae, thus takes its place among the "living fossils" (see p. 630). Its avail- 

 ability makes possible a study of the internal anatomy of primitive types 

 about which only what could be inferred from the characteristics of the fossil 

 shells was known previously. Preliminary studies on Neopilina have revealed 

 a well-developed internal segmentation, involving coelom, nephridia, gonads, 

 heart, and other structures. Students of the Mollusca are eagerly awaiting 

 the results of detailed anatomical studies, which promise to add new per- 

 spective to our views on moUuscan evolution. 



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