THE GASTROPODA 



is retracted, but is pushed back against the axis when the animal 

 is extended. 



The Gastropods are attached to their shells by the columellar 

 muscle (Fig. 45, co\ and withdraw themselves into their shells by its 



Fio. 68. 



Caecum. A, entire shell, left-side view, o, aperture ; .<*, septa ; sp, spire. (After de Folin.) 

 B, animal with truncated shell, e, eye ; /, foot ; m, mouth ; op, operculum ; sp, septum ; te, 

 tentacle. (Alter Plate.) 



contraction. This muscle is symmetrical .and horseshoe-shaped in 

 species with conical shells (Patella, Septaria, Capulus, etc.), but in 

 other species it is asymmetrical. It is oval in Haliotis, and its 

 insertion on the columella is nearly linear in coiled forms. The 



Fio. 69. 



Shell of Crucibulum, seen from below, showing 

 the inner whorl 6, concealed by the cap-like outer 

 whorl, a. (From Lankester.) 



Fio. 70. 



Cypraea europaea, dorsal 

 view, x 8. /, foot ; ma, 

 mantle ; sh, shell ; n, 

 pallial siphon ; t, tentacle. 



power of this muscle is often considerable : in Patella vulgata it can 

 resist a traction of fifteen kilograms. The columellar muscle is 

 naturally absent in forms without shells (Nudibranchs, Oncidium, 

 Vaginula), but is present in Testacella. 



3. Origin of Naked Forms. In many cases the oorders of the 



