STRUCTURE OF THE MOLLUSCA. 189 



The Calmaries have the body much elongated and fur- 

 nished with two terminal fins. Their internal shell is 

 horny ; but they have ten arms, two of them more elongat- 

 ed, like the cuttle-fishes, which they otherwise resemble in 

 structure. 



The Tetrabranchial Cephalopoda differ from those 

 spoken of, in having four in place of two branchiae, and in 

 their tentacula or arms being destitute of cups. Their body 

 is contained in the last cell of a large spirally convolute 

 shell, divided by transverse partitions into several cavities 

 or cells. Each of these partitions is perforated by a hole, 

 and the tube or siphon thus formed extends to the posterior 

 extremity or apex of the shell, and is lined by a membranous 

 contractile tube posterior to the body of the animal. The 

 siphon communicates with an external reservoir, and can be 

 distended with fluid, or emptied, so that the air which oc- 

 cupies the cells may be compressed or dilated, and thus in- 

 crease or diminish the specific gravity of the animal. The 

 Pearly Nautilus is the type of this group of animals, to which 

 also belongs the Ammonite family. 



CLASS II -PTEEOPODA. 



The Pteropodous Mollusca, fig. 2, are small marine ani- 

 mals organized for swimming, they having no organ by 

 means of which they can crawl, or even fix themselves to 

 objects. Their body is generally contained in part in a small 

 shell, fig. 2, a, but sometimes it is naked, and their neck is 

 furnished with two thin expansions resembling wings, fig. 2, 

 bb. The Hyale, for example, is of this kind. These ani- 

 mals abound in the seas of warm climates ; to which, how- 

 ever, the species are not restricted ; for some of them, as 

 Ctto borealis, are so numerous in the arctic seas, as at certain 

 seasons to furnish whales with their ordinary food. 



