CEPITALOPODS. 



471 



the branchiae or gills being four, in having no suckers, and in having 

 an external shell. The number of living species is extremely small — 

 for this group of animals belongs peculiarly to the earlier ages of oui 

 globe — is gradually becoming extinct, and presents in our days only 

 some species very rare and few, especially when we compare them 

 with the prodigious numbers of these beings which animated the seas 

 of the ancient world. In fact, the only living genus of the order is 

 that of Nautilus, the external shell of which has a singular resem- 

 blance in form to the internal shell of the genus Argotiaiita. 



In Nautilus the shell has a regularly convoluted form, the last 

 whorl being equal to all the others. It is divided internally into 



Fig. 320.- Nautilus pompilius (Linnseus), 

 showing the interior of the lower cell, to which the animal is fixed. 



numerous cells, formed by transverse partitions, concave in front and 

 perforated towards the centre, and forming a kind of funnel, which 

 gives passage to a respiratory syphon. 



In the last partition of the shell (Fig. 320) is the animal, covered 

 by its mantle, which lines the walls of the partitions. When it 

 contracts itself it is protected by a sort of triangular and fleshy hood. 

 Numerous tentacles, which are retractile, within sheaths, or " digita- 

 tions," corresponding to the eight ordinary arms of a cuttle-fish, and 

 some of them furnished with numerous lamellae, surround tlie head, 

 which is, besides, scarcely distinguished from the body. The head 

 bears two great projecting eyes, each planted upon a peduncle. 



Like Sepia and Octopus, the mouth of the Nautilus is armed with 

 mandibles, fashioned somewhat like the parrot's beak. The branchiae, 

 as we have seen, are four in number ; the circulatory system consists 



