Marine and Fresh-water. 73 



a name which was originally proposed by another 

 French naturalist, De Blainville, and though rather 

 longer than the older word is certainly more 

 distinctive. The term Lamellibranchiata simply 

 means " Platy gilled," and has reference to the fact 

 that in this class of shell-fish the function of respir- 

 ation is effected by means of two lamellar gills, 

 placed on each side of the body. 



The Lamellibranchiata or Conchifera form the 

 lowest group of the mollusca, and are characterized by 

 the absence of any distinctly differentiated part that 

 can represent a true head ; hence they are sometimes 

 referred to as the group of acephalous, or headless, 

 molluscs. They are familiar to every one, under 

 the form of oysters, scallops, cockles and mussels, 

 and are found on every coast and in every climate, 

 ranging from low water mark to a depth of 200 

 fathoms. 



All mollusca are, as their name implies, soft- 

 bodied animals {i7toliis^=soh) ; but this soft body is 

 usually defended by an external shell, whence the 

 popular name of "shell-fish," and whence too the 

 old scientific term of Testacea {testa~^^\it\\.) The 

 shell is composed mainly of carbonate of lime, or 

 as modern chemists sometimes prefer to call it, 

 carbonate of calcium or calcic carbonate ; but this 

 mineral-substance is associated with more or less 



