20 



MANUAL OF THE MOLLFSCA. 



attaclied is always indicated by scars 



to "wtich they are 

 (Fig. 14, a, a'). 



The border of the mantle is also muscnlar, and the place of 

 its attachment is marked in the shell by a line called the paUial 

 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 animal is withdrawn by retractor muscles also attached to the 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 chumc* 



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 7iavicella, concholepas, and the nautilus ; it becomes 

 deeper with age. In the spiral uniyalyes, the scar is less con- 

 spicuous, being situated on the columella, and sometimes divided, 

 forming two spots. It corresponds to the jDOsterior 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; 

 V, the umbo; I, the lunula ; c. cardinal tooth; 1 1', lateral teeth; a, anterior adductor ; 

 a ', posterior adductor ; p, pallial impression ; s, sinus, occipied by retractor of the 

 Bipbons. 



