446 



THE OCEAN WORLD. 



or tentacles. They differ from the first group chiefly in their more 

 numerous arms, which are quite tentaculiferous, having neither suckers 

 nor capsules, and hy having an external shell. The numher of living 

 species is extremely limited ; for this group of animals belongs pecu- 

 liarly to the earlier ages of our globe, is gradually becoming extinct, 

 and presents in our days only some rarer species, when we compare 

 them with the prodigious numbers of these beings which animated the 

 seas of the ancient world. In fact, the only living type of the order 

 is ^the nautilus, which has a singular resemblance in form to the 

 argonaut. 



The shell of this mollusc has a regularly convoluted form, the last 

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

 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 siphon. 



In the last cell of the 

 shell (Fig. 312) is the ani- 

 mal, covered by its mantle, 

 which covers the walls of 

 the cells. When it con- 

 tracts itself it is protected 

 by a sort of triangular and 

 fleshy hood. Numerous 

 contractile tentacles, re- 

 entering into the sheath, 

 Flg 3,2. N,m,, S p,. mP u,,, s( Lin n! eu S ), sh w 1 n B t h sm. some of them furnished 



terior of the lower cell, to which the animal is fixed. ^fa numerOUS lamellae, 



surround the head, which is, besides, scarcely distinguished from the 

 body. The head bears two great projecting eyes, planted upon a 

 peduncle. 



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

 mandibles, fashioned like the parrot's beak ; the branchiae are four in 

 number. The circulating system consists of a ventricle and auricle, 

 and the locomotive tube is protected in its whole length. The shell 

 is sscreted by the outer edge of the mantle, while its posterior ex- 

 tremity fashions the walls of the cells, which indicate the successive 

 growth of the individual. 



The siphon, which traverses all the chambers, receives and protects 



