THE CEPHALOPODA 



125 



the protoconch and are perforated in (or near) the centre 

 by an opening around which the septum extends like 

 a bottle-neck (Fig. 36, S.N.). At the opposite end from 

 the protoconch a considerable portion of the cavity of the 



FIG. 36. ORTHOCERAS. 



a, a', a", O. capax Barrande, Silurian, a, Cast of body-chamber and 

 three gas-chambers, part of test remaining (shown by fine striations) ; 

 a', median section of a similar broken specimen ; a", view of end 

 broken along a septum, b, O. decipiens Barrande, Silurian, cast 

 showing perfect body-chamber with constriction, septa more closely set 

 than in O. capax. c, O. capillosum Barrande, Silurian, median section 

 showing perfect body-chamber without constriction, gas-chambers 

 partly filled, and continuous siphuncle. (All after Barrande.) B.C., 

 Body-chamber; C, constriction; G.C., gas-chamber; 5, septum; 

 Siph., siphuncle ; S.N., septal neck ; Sut., septal suture ; T, test. 



shell is undivided, forming the body-chamber, lodging 

 the animal's body. The rest are gas-chambers, con- 

 taining (in Nautilus] a gas secreted from the blood, 

 resembling atmospheric air but with more nitrogen. 



