796 



ZOOLOGY 



sheath, the guard, produced into a horny or calcareous plate, the 

 pro-ostracum {pen.). In Sepia the spine- like projecting point 

 represents the guard, and the main sub- 

 stance of the shell is to be looked upon as 

 the pro-ostracum and phragmacone, the 

 septa of the latter being represented by 

 the calcareous lamellee. In the Squids 

 {e.g.jLoligo) the shell (Fig. 688, B) is long, 

 narrow, and completely horny; it corre- 

 sponds to the pro-ostracum, the phrag- 

 macone being entirely absent. 



In Octopus the shell is represented only 

 by a pair of vestiges with which muscles 

 are connected. In Argonauta there is no 

 shell in the male, but the female has an 

 external shell (Fig. 696) of a remarkable 

 character. This is a delicate spiral struc- 

 ture the internal cavity of which is not 

 divided into chambers. It is not secreted 

 by the mantle like the shells of other 

 Mollusca, but by the surfaces of a pair of 

 the arms ending in expanded disc-like 

 extremities, which become applied to its 

 outer surface (Fig. 689) ; its chief function is to carr)' the eggs. 

 An internal cartilaginous skeleton is present not only in 

 Sepia and Nautilus, as already described, but in all the Cephalo- 

 poda. Such an internal skeleton occurs in other groups — some 



Fig. (.'iliri.— Sliull of a Beleiu- 

 nite. u<J. guard ; 'iicn, 

 pro-ostracum ; p/i?-. phrag- 

 mocone. (From Nicholson 

 and Lydekker's Paht:oiito- 

 loiiy.) 



Fir;. O'JO.— Shell of Argonauto argo. 



Ch:rtopoda (p. 472), Crustacea, and Arachnida (p. 667) — but 

 attains a much more elaborate character in the present gi'oup than 

 in any other Invertebrates. 



