234 



ligament of the hinge, which constantly tends to open them 

 asunder. 



The animal has no head; the mouth is situated at the 

 anterior part of the body and is surrounded by fleshy lips. 



It opens into a short ossophagus which leads into a 

 stomach and intestine of various lengths, and terminates 

 in an anus. There is a large glandular liver for secreting 

 bile, which is poured by numerous ducts into the digestive 

 tube. 



Circulation is accomplished by arteries and veins, aided by 

 a symmetrical two-chambered heart ; the ventricle of which 

 in some genera surrounds a part of the intestine, the rectum. 



Respiration is performed by four equal-sized branchial 

 leaflets distinct from the mantle, and arranged symmetrically 

 on each side of the body. 



The nervous system consists of ganglia, situated at the 

 anterior and posterior parts of the body, from whence nerves 

 proceed to the labial organs, the adductor muscles, branchiae, 

 viscera, mantle, and foot. 



In the pecten a number of bright little ocelli, or 

 eyes, like small emeralds, are set round the free margin of 

 the mantle. These organs of vision are found in several 

 genera of the class,* as area, cardium, &c. 



The shell consists of the hardening material, the carbonate 

 of lime, deposited in an organised gelatinous cellular mem- 

 brane. The forms of the cells and structure thereof vary 

 in the different families. The animal membrane and earthy 

 matter are arranged in successive layers, the number of which 

 increases with the growth of the mollusc, the largest and 

 outermost being the last formed, as seen in amcula longi- 

 costata (fig. 164) from the lias. 



The shell consists of external fibrous and internal nacreous 

 layers. These two portions are often detached in fossil speci- 

 mens. The internal nacreous lamina receives the imprint of 

 the mantle, the insertion of the muscles, and forms the hinge 

 for uniting the valves. The preservation of this layer is of great 

 importance to the palaeontologist, as the impressions made 

 upon it by the organised parts of the body of the mollusc 

 reveal the organisation of its ancient fabricator, with as 



* Duvernoy du Spteme Nerveux des Mollusques. 



