388 



MOLL use A. 



308). Externally the shell of the former, coiled in a jjlane, 

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



Fia. 387.— Spmiki, with Internal shell. (After Owen.) 



is seen to be divided by jiartitioiis into numerons chambers which 

 increase in size tf)wards the aperture. r)nly 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 Xau- 



FiG. 388. — Diagram of shells, etc. of various cephalopods. (.\fter Lan;:r-) ^1, ScpiiT ^ 

 B, BehKcpia; C, BeJemnltes ; D, Ostraciteutlti.t : E, (iiiimastrii^his. n, anterior: i>, 

 posterior ; ju/f, phragmocone ; jjr, iiroostracuni ; r, rostrum ; s. siplton. 



tiloids and Ammonites — have a similar chambered shell: hut iu 

 other recent forms and in many extinct species the shell has 

 undergone a more or less complete degeneration. In iSpiriihi 

 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 Deoapoda 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 



