84 



MANUAL OF THE MOLLUSCA. 



Fig. 43. Nautilus pompilius in its shell.* 



The umh Ulcus is small or obsolete in the tj'pical nautili, and the whirls 

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

 are sometimes scarcely in contact. The last air-cell is frequently shallower 

 in propoi-tion than the rest. 



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

 a considerable extent ; they are surrounded by a circular fleshy Hp, external 

 to which are four groups of labial tentacles, 13 or 13 in each group, they 

 appear to answer to the buccal membrane of the calamary (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 form the 

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

 side, which is fiUed by the second pair of arms. The tentacles are lameUated 



* 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 ei/e is seen in the centre, covered by the hood (A) : t, 

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

 much contracted; «, nidamental gland ; a, c, air-cells and siphuncle; s, 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 ; ff, gizzard ; /, liver ; o, ovary. 



