198 



INVERTEBRATE ANIMALS. 



being known as the " pallial line " (Lat. pallium, a mantle), as shown 

 in fig. 138. 



As regards the shell of the Bivalves the following are the chief 

 points to be noticed. Each valve of the shell (fig. 138) is to be 

 regarded as essentially a hollow cone, the apex of which is turned 

 more or leas to one side. The apex of the valve is known as the 

 " umbo " or " beak," and is turned towards the mouth of the animal. 

 Consequently, the side of the shell towai'ds which the beaks are 

 turned is known as the " anterior " side, and it is usually much 

 shorter than the opposite or " posterior " side. The side of the shell 

 at which the beaks are situated is known as the " dorsal " margin ; 

 and here the valves are united to one another for a longer or shorter 



Fig. 138. — Left valve of Cytherea cliionc (after Wmtdward). A, Anterior margin ; 

 B, Posterior margin; C, Ventral margin or base; h Umbo; h Ligament; c C.ir- 

 dinal tuoth ; 1 1 Later.il teeth ; a Anterior adductor ; a' Posterior adductor ; 

 rp Pallial line ; s Pallial sinus caused by tlie retractor muscles of the siphons. 



distance along a line which is known as the "hinge-line." The 

 union between the valves is usually effected by means of inter- 

 locking parts or " teeth," and there is often a band of horny fibres 

 passing between the two valves just behind the beaks. In many 

 cases there is also a series of horny fibres placed perpendicularly 

 between the beaks, so as to Ije compressed when the shell is shut. 

 By the elasticity of these, and of the external ligament when pres- 

 ent, the valves of the shell are opened, without any effort of the 

 animal, simply by relaxing the adductor muscles. The valves are 

 shut again by the contraction of the adductor miiscle or muscles. 



As already said, the margin of the mantle leaves on the shell a 

 distinct impression — the " pallial line " — and by inspection of this. 



