84 



MANUAL OF THE MOLLIISCA. 



Fig. 43. Nautilus povipilius in its shell* 



The umhilicus is small or obsolete in the typical nautili, and the whirls 

 enlai'ge rapidly. In the palseozoic species, the whirls increase slowly, and 

 ai-e sometimes scarcely in contact. The last air-cell is fi'equently shallower 

 in proportion than the rest. 



Animal. In the recent nautilus, the mandibles are horny, but calcified to 

 a considerable extent ; they ai'e smTounded by a circidai' fleshy lip, external 

 to which ai'e four groups of labial tentacles, 12 or 13 in each gi'oup, they 

 appear to answer to the buccal membrane of the calamaiy (fig. 1). Beyond 

 these, on each side of the head, is a double series of arms, or brachial ten- 

 tacles, 36 in number ; the dorsal pair are expanded and united to foim the 

 hood, which closes the aperture of the shell, except for a small space on each 

 side, which is filled by the second pair of arms. The tentacles are lamellated 



* This woodcut and 18 others illustrating the tetrabranchiata, are the property of 

 Mr. Gray, to whom we are indebted for their use. Fig. 43 re])resents the recent 

 nautilus, as it appears on the removal of part of the outer shell-wall (from the specimen 

 in the British Museum). The eye is seen in the centre, covered by the hood (/<); t, 

 tentacles, nearly concealed in their sheaths ; /, funnel ; m, margin of the mantle, very 

 much contracted ; w, nidamental gland ; a, c, air-cells and siphuncle; «, portion of the 

 shell; a, shell-muscle. The internal organs are indicated by dotted lines; 6, bran- 

 chiae; h, heart and renal glands ; c, crop; g, gizzard; /, liver; o, ovary. 



