GASTROPODA. 283 
Similarly, part of the ventral surface is expanded into a 
separate muscular organ called the /oot. ‘his 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 Lame/hbranchiata 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 Lamellibranchiata 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 Aznulata, the lower marine types have larve, the 
pelagic ‘rochophore being a specially important type. 
Again, as in Amnulata, the terrestrial forms. and the 
highest marine forms (Cephalopoda) have eggs with quantities 
of yolk and an embryonic development. 
-Cxiass I.—GASTROPODA. 
Gastropoda are divided into two important sub-classes. 
The /sop/eura are few in number and small, but they are 
interesting from their worm-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 shige: 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 /u/monata stands rather apart owing to the 
adaptation to rial respiration and the loss of gills. It 
comprises the snails and the slugs. The rest are marine or 
