CLASS V. LAMELLIBRANCHIATA. 



519 



ORDER 2. GYMNOSOMATA. 



The animals of this order are distinguished by the absence of a shell, and the distinct separa- 

 tion of the head from the body. The species of 

 the genus Clio, belonging to this order, exist in 

 prodigious numbers in the Arctic and Antarctic 

 seas. So great, in fact, is their abundance, that 

 although they do not exceed an inch in length, 

 they furnish a great part of the food of the whale- 

 bone whales. They are usually of a beautiful 

 blue or violet color, tinged with red. In calm 

 weather they come in myriads to the surface to 

 breathe, but soon precipitate themselves toward 

 the bottom. Cuvier says that poi-tions of the Arctic seas are so glutted with these creatures 

 that a whale cannot open his mouth without ingulfing thousands of them. The most common 

 species are the C. Australis and C. Borealis. There is a larger species in the Indian Ocean of 

 a rose color. 



THE CLIO BORKALIS. 



TUB LAMP-bllELL. 



THE LINGULA ANATINA. 



Class IV. PAI.I.IOBRAI\€HIATA. 



This term is derived from the Latin pallium, a cloak, and branchice, gills, and refers to the 

 fact that in these animals the respiratory function is performed by the mouth, there being no 

 special branchial apparatus. They possess shells which are of peculiar structure, consisting of 

 flattened prismatic cells. They are always marine, and are found attached by the peduncle to 

 rocks, corals, and other submarine objects. The Lamp-Shell, Terebratula maxillata, has a 

 smooth convex shell, the animal attached by a peduncle. The Lingula anatina has a long pe- 

 duncle issuing from between the umbones. The valves are nearly equal, horny, and flexible. It 

 is found in the Indian Ocean. 



Class V. IiA]nEL.I.IBirANCIIIATA. 



This term is derived from the Latin lamella, a thin plate, and branchice, gills, and is used to 

 characterize the ordinary bivalve mollusca, which are usually inclosed in a bilobed mantle ; in 

 all cases the two sides of the mantle produce a calcareous shell consisting of two valves. These 

 animals are divided into two orders, the Siphonata and the Asiphonata. 



