328 INVERTEBRATE MORPHOLOQY. 



the two valves can be brought together, the ligament being 

 then compressed. The hinge is frequently complicated by 

 the development of tooth-like processes and corresponding 

 sockets so that the two valves may be firmly locked together. 

 Upon the inner surface of the valves are certain impressions 

 produced by the softer parts and of considerable value in 

 systematic conchology. A short distance from the margin of 

 each valve and parallel to it is a distinct line, the pallial 

 impression (Kg. 147, '^), produced by the attachment of the 

 muscle-fibres which bind the mantle-lobes to the shell. In 

 some forms, such as Anodon, this pallial line follows the shell 

 margin throughout its entire course, but in those genera 

 which possess well-developed and retractile siphons it is 

 deeply incurved in the posterior portion of its course. Oth«r 

 markings of the shells are produced by the insertion into 

 them of a number of muscles. The largest and most import- 

 ant of these are the adductor muscles of the shell (aa), large 

 muscles passing from one valve to the other, by their contrac- 

 tion overcoming the elasticity of the hinge-ligament and clos- 

 ing the shell. In the majority of forms there are two such 

 muscles, situated towards the anterior and posterior portion 

 of the body, but not unfrequently, as in Ostrea and Pecten, 

 but one, corresponding to the posterior adductor of other 

 forms, is present. In the immediate vicinity of the adductor- 

 impressions other smaller muscle-impressions are usually 

 observable, produced by the protractor and retractor muscles 

 of the foot and siphons. 



Although in the Pelecypod shell the two valves are typically similar 

 and symmetrical, yet in a number of cases a marked dissimilarity is found 

 in their shape. Thus in Ostrea the valve upon which the animal rests, 

 usually the left valve, is large and concave, while the other is smaller 

 and flattened, and a similar relation is found in other forms which he- 

 come temporarily fastened to rocks, etc. Occasionally additional cal- 

 careous plates are added to the usual shell, as in the boring mollusk P7ioZas, 

 in which three accessory calcareous plates are developed on the dorsal sur- 

 face of the body. In tlie Ship-worm, or Teredo, which bores extensively 

 into timber and is in some cases exceedingly destructive, the true shell- 

 valves are very small and situated at the anterior end of the body, and the 

 mantle projects backwards far beyond them and secretes a thin calcareous 

 tube which lines the interior of the passages excavated by the animal. A 



