GASTROPODA. 283 



Similarly, part of the ventral surface is expanded into a 

 separate muscular organ called the foot. This is usually 

 concerned with locomotion, but in the Cephalopoda the hind 

 part only assists locomotion, the front part becoming 

 modified into ingestive organs {cf. legs of Arthropoda). 



In all but the Lamcllibranchiata the buccal cavity contains 

 a peculiar toothed tongue or odontophore. The gills are 

 typically one pair of ctenidia, usually enveloped by the 

 mantle. 



The Mollusca are sometimes divided into two sub-phyla, 

 the Lameilibranchiata being contrasted with the other two 

 classes, but these also are so divergent that it is convenient 

 to keep them apart. 



The Mollusca do not invade the land with such success 

 as the Annulata. ' Only one class. Gastropoda., has terrestrial 

 representatives in the slugs and snails, and these are not 

 completely adapted for terrestrial life, for they revel in wet 

 and can only progress on a wet surface. 



The development of the phylum is very divergent. As 

 in the Annulata., the lower marine types have larvae, the 

 pelagic irochophore being a specially important type. 

 Again, as in Annulata., the terrestrial forms, and the 

 highest marine forms {Cephalopoda) have eggs with quantities 

 of yolk and an embryonic development. 



Class I. — Gastropoda. 



Gastropoda are divided into tw^o important sub-classes. 



The Isopleura are few in number and small, but they are 

 interesting from their w^orm-like character and the absence 

 of the torsion of other Gastropoda. Chiton is one of the 

 commonest types. A species about one inch long occurs 

 round our coast. It has several' dorsal shells and the gills 

 are also repeated. 



The Anisopleura comprise all the rest of the Gastropoda, 

 They have a trace of more or less dorsal torsion, supposed 

 to be the effect of a spiral shell. In most this also involves 

 the loss or reduction of one gill and one nephridium. 



The order of Pulmonata stands rather apart owing to the 

 adaptation to serial respiration and the loss of gills. It 

 comprises the snails and the slugs. The rest are marine or 



