MOLLUSC A PROPER. 



CHAPTER XLVI. 



LAMELLIBRANCHIA TA. 



Division II. Mollusca Proper. — This division includes those 

 members of the sub-kingdom Mollusca in which the nervous 

 system consists of three principal pairs of ganglia ; and there is 

 always a well-developed heart, which is never composed of fewer 

 than two chambers. 



The Mollusca proper may be roughly divided into two great 

 sections, respectively termed the Acephala and the £nceph»la 

 (or Cephalophora), characterised by the absence or presence of 

 a distinctly differentiated head. The headless, or Acephalous, 

 Molluscs correspond to the class Lamellibranchiata ; also dis- 

 tinguished, at first sight, by the possession of a bivalve shell. 

 The Encephalous Molluscs are more highly organised, and are 

 divided into three classes — viz., the Gasteropoda, the Pteropoda, 

 and the Cephalopoda. The shell in these three classes is of 

 very various nature, but they all possess a singular and com- 

 plicated series of lingual teeth ; hence they are grouped to- 

 gether by Professor Huxley under the name of Odontophora. 



Class I. Lamellibranchiata, or Conchifera. — The 

 members of this class are characterised by the absence of a 

 distinctly differentiated head, and by having the body more or 

 less completely protected in a bivalve shell. There are two 

 lamellar gills on each side of the body, the intestine has a 

 neural flexure, and there is no odontophore. 



The Lamellibranchiata are commonly known as the bivalve 

 shell-fish, such as Mussels, Cockles, Oysters, Scallops, &c., 

 and they are all either marine or inhabitants of fresh water. 



Though they agree with the Brachiopoda in possessing a 

 shell which is composed 'of two pieces or valves, there are, 

 nevertheless, many points in which the shell of a Lamelli- 

 branch is distinguishable from that of a Brachiopod, irrespec- 

 tive of the great difference in the structure of the animal in 



