84 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 
Fy I Af 
Fic. 68. 
r= Caecum. A, entire shell, left-side view. 0, aperture; se, septa; sp, spire. (After de Folin.) 
B, animal with truncated shell. e, eye; f, foot; m, mouth; op, operculum; sp, septum; Ze, 
tentacle. (Atter Plate.) 
contraction. ‘This muscle is symmetrical and horseshoe-shaped in 
species with conical shells (Patella, Septaria, Capulus, ete.), but in 
other species it is asymmetrical. It is oval in Hadiotis, and its 
insertion on the columella is nearly linear in coiled forms. The 
Fie. 70. 
; Hic: 169: Cypraea ewropaea, dorsal 
Shell of Crucibulum, seen from below, showing view, x 3. jf, foot; ma, 
the inner whorl b, concealed by the cap-like outer mantle; sh, shell; si, 
whorl, a. (From Lankester.) pallial siphon ; ¢, 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 borders of the 
