20 MANUAL OF THE MOLLUSCA. 
to which they are attached is always indicated by scars 
(Fig. 14, a, a’). 
The border of the mantle is also muscular, and the place of 
its attachment is marked in the shell by a line called the pallial 
impression (p); the presence of a bay, or sinus (s), in this line, 
shows that the animal had retractile siphons; the foot of the 
animal is withdrawn by retractor muscles also attached to the 
Fig. 14. Left valve of Cytherea chionc.* 
shell, and leaying small scars near those of the adductors 
(Fig 30*). 
The gasteropods withdraw into their shells when alarmed, by 
a shell-muscle, which passes into the foot, or is attached to the 
operculum ; its impression is horse-shoe-shaped in the limpet, 
as also in navicella, concholepas, and the nautilus; it becomes 
deeper with age. In the spiral univalves, the scar is less con- 
spicuous, being situated on the columella, and sometimes divided, 
forming two spots. It corresponds to the posterior retractors in 
the bivalves. 
Digestive System. This part of the animal economy is all- 
important in the radiate classes, and scarcely of less consequence 
in the mollusca. In those bivalves, which haye a large foot, the 
digestive organs are concealed in the upper part of that organ ; 
the mouth is unarmed, except by two pairsof soft membranous 
* Fig. 14. Cytherea chione, L., coast of Devon (original); h, the hinge ligament; 
u, the umbo; /, the lunule; c, cardinal tooth; ¢ t’, lateral teeth; a, anterior adductor ; 
a’, posterior adductor; p, pallial impression; s, sinus, occupied by retractor of the 
siphens. 
