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food of the molluscs, especially those which are fixed to 

 the rocks like the oysters, which depend for their sustenance 

 upon what the sea conveys into their shells. In common with 

 all the rest of the animal kingdom, the sub-kin^'dom of 

 the molluscs, is divided into classes, orders, sections, families, 

 genera aud species, and it is hardly necessary to remind you 

 that by assigning to every member of the animal kingdom, 

 from the largest to the must minute, and from the 

 highest to the lowest organization, its place in each 

 of these divisions, the naturalist is able to describe every 

 one by the peculiarities which characterize it, and which 

 distinguish it from every other one. Although the molluscs 

 have no hard or bony structure, a very large proportion of 

 them have their soft bodies pvotected by external shells. 

 Those which are thus enclosed, are called testaceous or 

 shelly molluscs, those without external shells are called 

 naked molluscs. The snail will be the most familiar type of 

 the one, and the slug of the other. A snail may be 

 called a slug in a shell, and a slug a 

 snail without a shell. The sub-kingdom of the molluscs 

 is divided into six classes, two of which are altogether 

 testaceous, three partly testaceous and partly not, and one 

 without any shell whatever. The first is that of the 

 Cephalopoda, or, as we may term it, the bead-footed, so- 

 called because their organs of motion are situated round 

 their heads. This class consists of both naked and testa- 

 ceous molluscs, the cuttle fish being the type of the one 

 group, and the nautilus that of the other. Of the cuttle 

 fishes, applying that term generally to the whole group, 

 there are four living families, consisting of numerous 

 genera, and amounting in the whole to about 200 species, 

 distributed in all parts of the world. Of the testaceous 

 Cephalopoda there are only two living genera, the nautilus 

 and the argonaut, or paper nautilu.s, or, to speak more 

 correctly, only the nautilus, the argonaut being excep- 

 tional ; the species of both together do not amount to a 

 dozen, but the testaceous cephalopoda are the beings of an 



