MOLLUSCA. GASTEROPODA 233 



the oesophagus, and the pleural and pedal placed below it ; 

 a visceral nerve-cord, which may bear ganglia, comes off 

 from the pleural ganglia, and forms a loop ventral to the 

 intestine. In some gasteropods (the Euthyneura) this 

 loop is simple, but in others (the Streptoneura) one side 

 is bent over so that the loop forms a figure of 8. Some 

 gasteropods are unisexual, others hermaphrodite. 



In the majority of the gasteropods a shell is secreted 

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

 is internal, but usually it is external and covers the visceral 

 mass. 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 coiled into a 

 spiral, generally screw-like, each coil being termed a whorl ; 

 in a few genera (e.g. Vennetus, Siliquaria) the whorls are 

 separated, but as a rule they are in contact (fig. 98), the 

 line between two contiguous whorls being known as the 

 suture (su). All the whorls, except the last, together form 

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

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

 frequently much larger — than the one preceding, and the 

 part of it farthest from the apex is called the base of the 

 shell. The spire varies in form in different genera and 

 species ; sometimes it is composed of a large number of 

 whorls, sometimes of few, and it may be long, short, or 

 depressed; occasionally all the whorls are in one plane. 

 The angle of the spire (spiral angle) consequently varies ; 

 this is measured by lines drawn from the apex to the base 

 of the shell on opposite sides of the exterior of the whorls. 

 The coiling of the shell is usually dextral ; so that when the 

 apex of the shell is pointed away from the observer (as in 



