MOLLUSCA. GASTEROPODA. 151 



posterior, but as a rule it is placed near the opening of 

 the gill chamber. Placed just within the mouth on the 

 floor of the cavity is a dental apparatus, known as the 

 odontophore ; this consists of a chitinous ribbon, on which 

 numerous teeth are placed in rows, it is moved by means 

 of muscles and serves as a rasping organ. The arrange- 

 ment of the teeth varies in different genera and is of 

 considerable importance in the classification of recent 

 gasteropods, but since the odontophore has never been 

 definitely recognised in the fossil forms, it is of no service 

 to the palaeontologist. The nervous system consists of 

 three pairs of ganglia united by cords. Some of the 

 gasteropods are unisexual, others hermaphrodite. 



In the majority of the gasteropods a shell is present, 

 secreted by the mantle ; in a few forms, as for instance 

 the slugs, it is internal, but usually it is external. The 

 shell, except in Chiton, and its allies, consists of a single 

 piece, and is hence said to be univalve. In the limpet 

 (Patella) it has the form of a hollow cone. But in most 

 cases it consists of a long tube, open at one end, and 

 tapering to a point at the other. This tube is generally 

 coiled into a spiral, each coil being termed a whorl ; in a 

 few genera (e.g. Vermetus, Siliquaria) the whorls are 

 separated, but as a rule they are in contact (fig. 47), the 

 line between two contiguous whorls being known as the 

 suture (su). All the whorls except the last one form 

 together the spire (S) of the shell, the point of which is 

 termed the apex (a). The last whorl is nearly always 

 larger than the preceding, frequently very much so, and 

 the part of it farthest from the apex forms the base of the 

 shell. The form of the spire varies in different genera 

 and species, sometimes it is composed of a large number 

 of whorls, sometimes of few, and it may be elongated, 



