20 



MANUAL OF THE MOLLUSCA. 



to which they are attached is always indicated by scars 

 (Fig. 14, a, aO- 



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 si7ius (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 animal is withdrawn by retractor muscles also attached to the 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 chione.* 



shell, and leaving 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 have a large foot, the 

 digestive organs are concealed in the upper part of that organ ; 

 the mouth is unarmed, except by two pairs of soft membranous 



* Fig 14. Cytherea chione, L., coast of Devon (original); h, the hinge ligament; 

 tt, the umbo ; I, the lunule ; c. cardinal tooth ; 1 1', lateral teeth ; a, anterior adductor : 

 a ', posterior adductor ; p, pallial impression ; s, sinus, occupied by retractor of tho 

 siphons. 



