MOLLUSC A PROPER. 



CHAPTER XLIV. 

 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 Encephala 

 (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 (Pelecypoda}. 

 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 one or 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 (small acces- 

 sory plates are present in Pholas, &c.), there are, nevertheless, 

 many points in which the shell of a Lamellibranch is distin- 



