468 MARINE INVERTEBRATES 
the septa are perforated, and through the opening runs a tube or 
prolongation of the body, the siphuncle, 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 
branchie 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 Dibranchiata are 
divided into two orders: the Octopoda, 
which have eight arms, and comprise 
the Octopus and Argonauta; 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 
Argonauta argo, female removed from shell. prehension. 
ane C Usd dH RAAT ETL IONS 
