CEPHALOPODA. 



213 



towards the keel. It swims by the jets of water emitted from the 

 funnel, and crawls upon the sea-bottom, head downwards, carrying 

 its shell on its back. The male Argonaut (fig. 150) is only about an 

 inch in length, has no shell, and has all its arms alike, except the 

 one which is metamorphosed into the " hectocotylus." The Poulpes 



Fig. 151. — ArgO}iauta argo, the Paper Nautilus, female. The animal is represented in 

 its shell, but the webbed dorsal arms are separated from the shell which they 

 secrete, and which they ordinarily embrace. 



(Octopi) are universally distributed in the seas of both temperate 

 and tropical regions. They are the " polypi " of Homer and Aris- 

 totle, and are voracious animals inhabiting rocky shores. 



The Decapoda are chiefly found in the open sea, often in enormous 

 numbers, and the best known are the Calamaries and Squids. The 

 body is elongated, and is always furnished with lateral fins, with 

 which they swim actively. The shell is internal, hidden in the 

 mantle, and differs considerably in different members of the group. 

 In the common Calamaries it forms a horny pen, with a, central 

 shaft and two lateral expansions or wings (fig. 152, b). In the 

 genus Sepia, the skeleton is calcareous, and is sometimes called the 

 " cuttle-bone " (fig. 152, a). In the curious Spirul(e of i-ecent seas, 

 the shell is rolled up into an open spiral, and is divided by calcareous 

 partitions into a succession of chambers (fig. 152, c), which are placed 



