MOLLUSKS IN GENERAL 



225 



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'Foot 



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often called sea slugs or nudibranchs, usually found in beds of seaweeds, 

 which exhibit varied forms and a wide range of colors. 



262. Scaphopoda. — This class contains a limited number of moUusks 

 which are found at moderate depths in the sea, buried in the mud at the 

 bottom. They have a mantle which is tubular in form and which secretes 

 a tubular shell open at both ends and larger at one end than at the other. 

 This shell is curved like the tusk of an elephant, so these are sometimes 

 called tusk shells (Fig. 135). The foot protrudes through the larger end 

 and is used for boring in the mud. 



263. Pelecypoda. — This class, also known as the Lamellibranchiata, 

 is represented in both fresh and salt water and includes forms which 

 are similar to the fresh-water mussels already described. 

 They possess no head and have a bivalve shell. Among 

 the various types of pelecypods are mussels, scallops, 

 oysters, clams, and the shipworm. Many of the marine 

 forms in this group are permanently attached to firm 

 objects on the bottom or along shore, from which objects 

 they can be dislodged only with great difficulty. This 

 attachment may be by horny threads, forming a hyssus, as 

 in the marine mussels, or may be due to the disappearance 

 of a part of the under valve of the shell and the firm 

 adherence of the animal's body to the substratum through 

 the hole so formed, as in the case of some oyster-like forms. 

 Such forms do not move about in the adult stage, and the 

 foot may be undeveloped. Scallops have numerous eyes 

 along the margins of the mantle. 



In many bivalves a blind sac or caecum connected 

 with the intestine secretes a rodlike body, or crystalline 

 style, the function of which is unknown. 



264. Cephalopoda. — The cephalopods make up a 

 group which in many respects is very much higher than 

 any other class of mollusks. They are all marine. They 



have the dorsoventral diameter of the body greatly increased and 

 the anteroposterior diameter greatly reduced. They may even become 

 so flattened anteroposteriorly that ends and surfaces change places. 

 What appears to be the anterior or head end when the animal 

 is in its normal position is really the ventral surface, and the 

 apparent posterior or tail end is the dorsal surface; the real anter- 

 ior end is the upper surface and the real posterior end the lower 

 surface (Fig. 136). In addition to this marked change in the axes the 

 foot is concentrated about the "head" and has become divided into a 

 number of tentacles situated in a ring about the mouth. These tentacles 

 have a great many sucking discs. The relationship of mantle and shell 

 to the body differs in different types of cephalopods. 



Fig. 135.— 

 A tusk shell, 

 Dentalium preti- 

 osum Sowerby, 

 from Puget 

 Sound. Natural 

 size. 



