MOLLUSC A : LAMELLIBRANCHIATA. 251 



MOLLUSC A PROPER. 



CHAPTER XLVI. 



LAMELLIBEANCHIA TA. 



DIVISION II. MOLLUSCA PKOPER. 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 welt 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 Lamellibranclimta ; also distin- 

 guished, 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 together 

 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 com- 

 pletely 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 Lamellibranch 

 is distinguishable from that of a Brachiopod, irrespective of the 

 great difference in the structure of the animal in each. The 

 shell in the Brachiopoda, as we have seen, is rarely, or never, 

 quite equivalve, and always has its two sides equally developed 

 (equilateral) ; whilst the valves are placed antero-posteriorly 

 as regards the animal, one in front and one behind, so that 



