CLASS V. LAMELLIBRANCHIATA. 
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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, 
i^^^^ ^^^^ 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 
THE CLIO BOREALis. wcathcr they come in myriads to the surface to 
breathe, but soon precipitate themselves toward 
the bottom. Cuvier says that portions 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 0. Borealis. There is a larger species in the Indian Ocean of 
a rose color. 
THE LAMP-SHELIi. THE LINGULA ANATINA. 
Class IV. FAI.l,IOBRAI¥€HIATA. 
This term is derived from the Latin pallium, a cloak, and branckice, 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, Terehratula 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. I^AMElil^IBRANCHIATA. 
This term is derived from the Latin lamella, a thin plate, and branckice, 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 iSipkonata and the Asiphonata. 
