23.2 MOLLUSCA. GASTEROPODA 



mass and the atrophy of some of the organs on one side of 

 the body. There is a distinct head, which bears one or 

 two pairs of tentacles, and usually also eyes. On the 

 ventral surface of the body is the foot ; this is usually 

 large and sole-like and used for crawling, but in the 

 Heteropods it is in the form of a flattened fin, and in the 

 Pteropods it is wing-like. The mantle is never divided 

 into two lobes. Respiration takes place in some cases 

 through the skin, but generally by means of a lung-cavity 

 or by gills ; the latter are placed in a sac formed by the 

 mantle ; sometimes they are present on both sides of the 

 body, but usually the original left gill has disappeared. In 

 some forms the mantle, at the opening of the gill-sac, is 

 produced into a tube, known as the siphon, by means of 

 which water passes to the gills. The heart is on the 

 dorsal surface, and consists of a ventricle and usually one, 

 but in some cases two auricles. In many forms the gills 

 are placed in front of the heart, but in others behind it. 

 The mouth is at the anterior end of the body ; the anus 

 is occasionally posterior, but as a rule it is placed near the 

 opening of the gill chamber. On the floor of the cavity of 

 the mouth is a dental apparatus, known as the odontophore : 

 this consists of a cartilaginous and muscular ridge on 

 which rests a chitinous ribbon (the radula) ; the radula 

 bears numerous teeth placed in rows, and serves as a 

 rasping organ. The arrangement of the teeth varies in 

 different genera and is of considerable importance in 

 classification, but since the radula has never been definitely 

 recognised in fossil forms, it can only be used by the 

 palaeontologist in the case of genera which have existing 

 representatives. The nervous system consists of ganglia 

 which are connected by nerve-cords. Typically there are 

 three pairs of principal ganglia — the cerebral placed above 



