MOLLUSCA CEPHALOPODA. 95 



CHAPTER IV. 



MOLLUSCA AND MOLLUSCOIDA. 



SUB-KINGDOM IV. Mollusca. 



CLASS 1. Cephalopoda. 



128. The Cephalopoda (Gr. Tcepliale, the head; poda, 

 feet) is the highest class of Mollusca. It includes the 

 cuttle-fish, the octopus or poulpe, the calamary or squid, 

 the paper-nautilus, the pearly-nautilus, the extinct am- 

 monites, belemnj.tes, &c. 



This class is characterized by having the organs of 

 locomotion, in the form of arms, arranged in a circle 

 around the mouth. These arms are usually furnished 

 with sucking cups, and are eight or more in number. 

 The body is covered with a loose mantle, which opens in 

 front, on the lower part of the body, to admit water into 

 the gill chamber. The gills are plume-like, and are 

 either two or four in number. They are placed within 

 the mantle. When the water in the gill chamber has 

 been deprived of its oxygen, by the process of respira- 

 tion, it is expelled through a " funnel " placed under the 

 head. The reaction produced by the escape of this water 

 causes the animal to move in the opposite direction. 



The Cephalopoda are all carnivorous animals, and live 

 in the sea. They walk along the sea bottom by means of 

 their long arms ; or swim, partly by the aid of a fin with 

 which the extremity of many of their bodies is furnished, 

 and partly by the reaction caused by the escape of water 

 through the funnel. 



129. Skeleton. The greater number of living cephalo- 

 pods are unprotected by any external shell. Most of 

 these, however, possess an internal shell, but this is 

 sometimes rudimentary. 



