DISTRIBUTION INTO ORDERS. 71 



laminse, like the teeth of a comb^ and are contained 

 within a cavit}^ in the upper part of the neck. Some 

 of the tribe are carnivorous ; others, herbivorous. The 

 former are universally distinguished by a siphon, or 

 siphonal appendage passing out of the basal channel of 

 the shell, for the purpose of conveying water into the 

 branchial cavity. 

 II. PuLMOBRANCHiATA. — Tliis Order includes the whole 

 of the pulmoniferous, or air-breathing MoUusca. Their 

 respiratory organs form a kind of vascular net-work, 

 lining a cavity in the back ; while the mantle is perfo- 

 rated in that part which covers it, the orifice being 

 furnished with a small fleshy valve, which the animal 

 opens and shuts at will. 



III. Pleurobranchiata. — Distinguished by the gills being 

 situated on the right side. 



IV. Cervicobranchiata. — With gills situated in a special 

 ca\dty in the neck. 



Y. Cyclobranchiata. — Gills placed in a circle round the 

 edge of the body. 



VI. Cirrobranchiata. — Cirrous, or hair-like gills. 



VII. Nucleobranchiata. — Comprising a class of moUusks 

 which exhibit a very distinct peculiarity of organization. 



