T7PE MOLLUSC A. 



361 



that finally nothing is left but a plate imbedded in the man- 

 tle, formed entirely of chitinous material and destitute of any 

 calcareous substance. This forms the so-called " pen " of 

 such forms as Loligo and Ommastrephes (Fig. 161, D), a slight 



Fig. 161. — Diagrams op Shell of A, Sepia; B, Belemnites (fossil); 0, Os- 

 tracoteutlds (ia&&i\); and -D, 0TOmasir«p7j«s (after Lang). 

 ph = phragmocone. r = rostrum. 



pr = pro5stracon. « = sipuncle. 



thickening of the dorsal end of it in some forms representing 

 the remains of the phragmacone. 



In the Octopoda the eight arms are practically equal in 

 length, and the body is more massive than in the Decapods 

 and less suited for active swimming. TJie visceral hump is a 

 more or less globular mass, destitute as a rule of lateral fins, 

 and in all forms a shell comparable to that of the Decapods 

 is wanting. In the females of Argonauta, however, a non- 

 chambered calcareous shell is present, to which the body of 

 the animal does not closely adhere, but which is held in posi- 

 tion by the broad plate-like anterior arms which embrace it. 



