CHAPTER XXXVIII 

 MOLLUSKS IN GENERAL 



Mollusca is a phylum more numerous in known species than any 

 other except Arthropoda. The mollusks are exceedingly varied in form 

 and structure and represent a great difference in grade of organization 

 between the lowest and highest. Indeed, the whole phylum may be 

 considered as representing a long line of descent from wormlike ancestors. 

 They have several features in common which distinguish them from all 

 other animals. One of these is the presence of a mantle, which varies 

 in form in different classes. Since the shell is secreted by the mantle, 

 its form varies accordingly. The mantle cavity, which is the respiratory 

 cavity, also varies in form and in the character of the respiratory organs 

 contained in it. Another characteristic structure of mollusks is the 

 ventral muscular foot, which is sometimes absent, as in the oyster. This 

 foot is typically not an appendage, since in most cases it is merely a 

 portion of the muscular wall of the body modified to form a smooth sur- 

 face which serves in locomotion. In the Cephalopoda, however, the 

 foot is modified in such a manner as to produce several long mobile 

 tentacles provided with suckers, which are truly appendages. 



259. Classification. — The phylum Mollusca (mol lus' ka; L., mol- 

 luscus, soft) is divided into five classes according to the characters of 

 foot, mantle, shell, and respiratory organs. The classes are as follows: 



1. Amphineura (amfinu'ra; G., amphi, on both sides, and neuron, 

 nerve). — The chitons. 



2. Gastropoda (gas trop' o da; G., gastros, stomach, and podos, foot).^ — 

 The snails. 



3. Scaphopoda (ska fop' 6 da; G., skaphe, boat, and podos, foot). — 

 The tusk shells. 



4. Pelecypoda (pel e sip' o da; G., pelekys, hatchet, and podos, foot). — 

 The bivalve mollusks. 



5. Cephalopoda (sef al op' o da; G., kephale, head, and podos, foot). — 

 The squids, octopuses, and nautiluses. 



260. Amphineura. — The Amphineura are clearly the most primitive 

 of the mollusks, reminding one in some respects of the worms. They 

 have a body which is flattened and elongated. In some of these forms 

 there is no shell, but the dorsal surface is covered by a soft mantle con- 

 taining many small limy spicules (Fig. 132 D). In this case the foot 



lies in a groove on the ventral surface. In other forms known as chitons 



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