200 



INVERTEBRATE ZOOLOGY 



Gonad 



•Sfowach 



dencies in development. But little is known of the habits and 

 development of Nautilus, for though the empty shells are cast 

 ashore in quantities in the Pacific and Indian oceans the animal 

 is rarely found alive. 



Subclass DiBRANCHIA 



In this subclass are included those cephalopods having two 

 branched gills in the mantle cavity, two nephridia, two auricles, 

 highly organized eyes, and eight or ten arms bearing suckers or 

 hooks. On the basis of the number of arms, two orders are 



recognized, namely, Decapoda — 

 with ten arms (devilfishes and 

 Argonauta) — and 0<}topoda — 

 with eight arms (squid, cuttlefish 

 and Spirula). 



Some sort of shell is usually 

 present, in the form of an internal, 

 coiled, chambered shell as in 

 Spirula; a long, horny pen of 

 purely organic matter as in the 

 squids (Loligo); or a highly cal- 

 careous plate known as the 

 "cuttlebone" of the cuttlefish 

 (Sepia, Fig. 94). In the argo- 

 nauts, the female is provided 

 with a thin, single-chambered 

 shell, but the male argonauts 

 and all other representatives of 

 the order typically lack a shell. 

 An ink sac (Fig. 94) is usually 

 present. The powerful, muscu- 

 lar arms are used both in swim- 

 ming and in the capture of prey. A constriction separates the 

 head from the body proper and marks the anterior boundary of 

 the mantle. At this level, on the ventral surface occurs the 

 respiratory opening. 



Man He 



Fig. 94. — Diagram of median section 

 of Sepia. {From Lang). 



Outline of Classification 



Phylum Mollusca. — Triplol^lastic; iinsegmented; coelom greatly reduced; 



shell or shell gland at least during larval development. 



1. Class Amphineura. — Nervous system not concentrated; shell when 

 present composed of eight transverse calcareous plates; marine. 



