108 ZOOLOGY. 



148. Development. The sexes are usually distinct, 

 but are united in the sea-slugs. The young, when hatched, 

 have a shell in all cases. The naked molluscs soon cast 

 this off. The young of the water-breathing species are 

 furnished with a pair of ciliated fin-like expansions, 

 resembling the wings of the Pteropoda, by which they 

 are enabled to swim with great facility. This is one of 

 many instances, in which the larval form of one group 

 resembles the permanent form of another. 



CLASS 4. Lamellibranchiata. 



149. The Lamellibranchiata (Lat. lamella, a plate; 

 Gr. branchia, gill) are so called, because they are pro- 

 vided with two gills or branchise on each side of the body, 

 in the form of membranous plates or leaves. They are 

 also called Conchifera (Lat. concha, a shell ; fero, I bear), 

 and sometimes Acephala (Gr. a, without; cephale, the 

 head). They are always provided with bivalve shells. 

 These may be distinguished from the bivalves of the 

 Brachiopoda, by being usually equivalve and inequi- 

 lateral; whereas, the brachiopod bivalves are always 

 equilateral, and never equivalve. They have a mouth, 

 but no distinct head. The lingual ribbon, or odonto- 

 phore, found in the higher molluscs, is absent. 



The Lamellibranchiata is a very numerous class, in- 

 cluding all the bivalve shell-fish ordinarily met with. 

 They are all aquatic. Most of them inhabit the sea, but 

 a few live in fresh water. Mussels, cockles, oysters, and 

 scallops are familiar examples. 



150. Skeleton. The mantle, instead of surrounding 

 the animal like a sack, as in the Gasteropoda, is divided 

 into two halves, or lobes, which are placed on the right 

 and left sides of the body. Sometimes the mantle lobes 

 are unconnected ; at other times they are united, except 

 at two points, where there are openings for the passage of 

 the foot and the breathing siphons. Each lobe secretes a 



