CEPHALOPODA. 267 



The pearly nautilus is the only living representative of this 

 group. It occurs in the East Indian seas, and, while the 

 shells are very common, the animal is very rare in museums. 

 In geological times allied forms were very abundant, and 

 are known as Ammonites (with tightly coiled shells), and 

 Orthoceratites (with straight shells), etc. 



ORDER II. DIBRAKCHIATA. 



These have two gills (one pair), and long, sucker-bearing 

 arms. An ink-sac is always present. The order is sub- 

 divided into the OCTOPODA, in which there are eight arms, 



FIG. 123. Octopus bairdii. From Verrill. One arm on the right side is 

 modified for purposes of reproduction. 



and the DECAPODA, in which the number is increased to ten 

 by the addition of a pair of longer arms. In the Octopoda 

 there is no internal shell, and the body is saccular. Here 

 belong the octopus, poulpes, etc., as well as the paper nau- 

 tilus, which does not sail with its shell as a boat, and its 



