284 



PHYLUM MOLLUSCA 



CLASS n— GASTROPODA 



The head is moderately well developed, with tentacles and eyes ; 

 the foot is more or less flattened dorsoventrally, and the visceral 

 mass has undergone the torsion described above (p. 266) so that 

 there is some degree of asymmetry. There is usually a shell in a 

 single piece and a radula is present. The snail illustrates these 

 general characters, but is atypical in that it and its relatives 

 which live on land or in fresh water (the Pulmonata) have lost 

 some features and developed new ones. The majority of gastropods 

 are marine, and have gills (ctenidia) in the mantle cavity and a 



trochophore larva. In many the 

 sexes are separate. The limpet 

 Patella has a shell with very slight 

 coiling and is herbivorous ; the 

 dog-whelk, Buccinum, like a large 

 5th shell poi^^^^ snail, is carnivorous, 

 plate 



CLASS LIl—SCAPHOPODA 



This is a small order of bi- 

 laterally symmetrical molluscs 

 with a tubular shell open at both 

 ends. That of Dentalium is com- 

 FiG. 212. — Chiton, dorsal view. mon ou the shore. 



eyes 



CLASS IV—LAMELLIBRANCHIATA 



These are bilaterally symmetrical and compressed animals with 

 a large mantle in two flaps. Each of these secretes its own portion 

 of the shell, so that this is bivalved, the two parts being connected 

 by a hinge. This bivalve construction determines, or is determined 

 by, the rest of the arrangement of the body, and the whole mode 

 of life ; there are special muscles to close the shell, and an elaborate 

 arrangement of cilia on the gills and mantle for feeding on small 

 particles. The labial palps border the mouth. The swan mussel 

 shows the general characters, but most members of the class are 

 marine, have the sexes separate, and have a trochophore larva. 



