PHYLUM MOLLUSCA 



261 



gills, or through the mantle. Eolis (Fig. 187) and Dendronotus 

 are common genera. 



The shelled marine Gastropoda usually breathe by means of 

 gills. In Sycotypus, for example, there is a trough-like extension 

 of the collar, the siphon, which leads a current 

 of water into the mantle cavity where the gill 

 is situated. The direction of this current of 

 water prevents contamination by the faeces 

 and excretory products. 



4. Class III. Scaphopoda 



This class contains only a few aberrant 

 marine mollusks called tooth shells. The 

 mantle forms a tube around the body and 

 secretes a crescent-shaped tubular calcareous 

 shell larger at one end than at the other. 

 Both ends of the shell are open. The foot 

 (Fig. 188, /), which is used for boring in the 

 sand, can be protruded from the larger 

 anterior aperture. The head is rudimentary, 

 but a radula is present. Eyes and a heart 

 are absent. The sexes are separate. Ex- 

 ample: Dentalium (Fig. 188). 



Fig. 188.— ASca- 

 phopod, Dentalium. 

 a, anterior aperture 

 of mantle ; /, foot ; 

 g, genital gland; 

 k, kidney ; I, liver. 

 (From the Cam- 

 bridge Natural His- 

 tory, after Lacaze- 

 Duthiers.) 



5. Class IV. Pelecypoda 



The Pelecypoda or Lamellibranchiata, 

 as they are often called, are the mussels, 

 clams, oysters, and other bivalves. They 

 are simple in structure and therefore favorite mollusks for lab- 

 oratory dissection (pp. 243 to 251), but are probably less prim- 

 itive than the Gastropoda. They do not possess a head or 

 radula. The mantle is bilobed and secretes a bivalve shell. The 

 gills are usually lamellate. 



The Pelecypoda are all aquatic and mostly marine. They 



