468 



MARINE INVERTEBRATES 



the septa are perforated, and through the opening runs a tube or 

 prolongation of the body, the siplmncle, which extends to the tip of 

 the shell. The chambers are filled with gas. The animal in the 

 course of its growth moves forward into a newly formed chamber 

 and builds a new septum, closing the cavity last occupied. Nau- 

 tilus lives among the coral reefs of the southern Pacific. Its 

 four or five species are the remnants of a once very extensive race 

 of cephalopods. The fossil remains of many species of Nautilus, 

 together with various other genera of shell-bearing cephalopods, 

 indicate that this group has seen its best days. The dibranchiate 

 genera, however, appear to have reached their maximum at the 

 present day. 



SUBCLASS DIBRANCHIATA 



The second subclass, the Dibranchiata, is characterized by two 

 branchiae and two auricles. The main part of the foot is divided 

 into eight or ten long arms provided with numer- 

 ous suckers arranged in from one to four rows 

 on the ventral side of the arms. They have 

 also an ink-sac. The DibrancMata are 

 divided into two orders: the Octopoda, 

 which have eight arms, and comprise 

 the Octopus and Aryonauta ; and the 

 Decapoda, which have ten arms, 

 and comprise Spirula, Omma- 

 strephes, Sepia, and Loligo. 

 In these animals two of 

 the ten arms are longer 

 than the others, and these 

 tentacular arms have 

 suckers only on their 

 broadened, club-like 

 ends, they are kept 

 retracted within 

 grooves, one on each 

 side of the head, ex- 

 cept when needed for 



Aryonauta. argo, female removed from shell. 



prehension. 



