THE PHYLUM MOLLUSCA 



The Mollusk Body Plan 



The primitive, generalized ancestor of the modern mollusks was probably 

 an animal with a simple head bearing the mouth, and with a body consisting 

 of a flattened foot surmounted by a visceral mass containing the organs 

 (Fig. 13.21). The visceral mass, we may assume, was covered dorsally by a 

 shell lined by a mantle. Within a posterior cavity enclosed by the overhang- 

 ing mantle were a pair of gills and, near the anus, the paired openings of 

 nephridia and reproductive organs. The visceral mass probably contained a 

 small coelom, a pair of digestive glands, a pair of nephridia, an open cir- 

 culatory system, a pair of reproductive organs, and a nervous system com- 

 posed of three pairs of ganglia with their connectives. The characteristics 

 of the fossilized shells of some of the earliest known mollusks, dating from 

 the Cambrian (see Fig. 20.1, p. 617), are in agreement with such an inter- 

 pretation of the features of forms ancestral to modern types. Although the 

 soft parts of these ancient mollusks are unknown, their characteristics may be 

 inferred from the features of the shells, muscle scars, etc. 



The fundamental characteristics of the ancestral type are retained to some 

 degree in all the modern descendants of this generalized ancestor. The 

 phvlum Mollusca exemplifies particularly well the principle of adaptive radia- 

 tion, which leads to the production of a number of distinct types, through 

 evolution in diflferent directions from a primitive stem form. Each type so 

 produced is generally adapted to the conditions of life in a particular type 

 of habitat. Thus the chitons are little modified in their general features; the 

 pelecypods have become bivalved and laterally compressed in adaptation to a 

 burrowing existence. The elongated scaphopods represent a diflferent set of 

 adaptations to burrowing life. The gastropods have undergone innumerable 



Pericardial cavity 

 ^,'-'"'0^ \--" ^^^/ ^~X/*\^~\/' Nephridium 



Cerebral ><;^^ V- ^\ IvTu^^^^^^^^^y 



ganglion /-^^^ "^ ~~""~~~~-~-/^~^!iv ^C\^^ \\V^^\\. 



Mouth-^^-"^ ^ jC ^ \^ Excretory 



ganglion \ _]^;;;ii,- — 



ganglion 



Fig. 13.21. Hypothetical ancestral type showine; features which may have been characteristic 

 of the forms from which all mollusks have evolved by adaptive radiation. (Redrawn from P. 

 Pelseneer in E. Ray Lankester, A Treatise on ^oology. Part 5, 1906.) 



393 



