388 



MOLL USC A. 



398). Externally the shell of the former, coiled in a plane, 

 resembles that of certain snails like Planorbis; but on section it 



Fio. 387. Spirula, with internal shell. (After Owen.) 



is seen to be divided by partitions into numerous chambers which 

 increase in size towards the aperture. Only in the last is the 

 animal situated: the others are filled with air. Each partition 

 has a small opening, and through these runs a strand of tissue, the 

 siphuncle. Among the fossil cephalopods many forms the Nau- 



FIQ. 388. Diagram of shells, etc. of various cephalopods. (After Lang.) A, Sepia; 

 B, Belosepia; C, Belemnites ; D, Ostracoteuthis ; E, Ommaiftrephe*. a, anterior; p, 

 posterior ; ph, phragmocone ; pr, proostracum ; ?, rostrum ; s, siphon. 



tiloids and Ammonites have a similar chambered shell; but in 

 other recent forms and in many extinct species the shell has 

 undergone a more or less complete degeneration. In Spirula 

 peronii (the animals of which are extremely rare, the dead shells 

 common) there is a similar chambered shell, buried in the mantle 

 (fig. 387). In the Decapoda the equivalent of the shell is com- 

 pletely concealed in the back of the animal. In the Sepias it is 

 a lamellar calcareous structure, the well-known cuttle bone; in the 



