NAUTILUS. 47 



The dorsal pair of tentacles is considered to be expanded, 

 forming the hood which closes the aperture of the shell ; 

 the tentacles are lamellated on their inner surface, and 

 are retractile within sheaths which correspond to the eight 

 ordinary arms of the cuttle-fishes. Besides these, there 

 are four ocular tentacles, which are supposed by Professor 

 Owen to be instruments of sensation, like the tentacles of 

 Doris and Aplysia ; there are, moreover, four groups of 

 labial tentacles, twelve or thirteen in each group, which 

 appear to answer to the buccal membrane of the dibran- 

 chiate Cephalopods ; the mandibles are surrounded by a 

 fleshy lip. The respiratory funnel, or siphuncle, is formed 

 by the folding of a thick muscular lobe ; behind the hood, 

 or foot-like appendage, it forms an expansion which covers 

 the black part of the shell. The habits of the Nautilus are 

 not well known ; most probably, like the Octopus, it feeds 

 upon Crustacea, and crawls along the bottom of the sea, 

 seeking concealment in rocky places; the tips of the parrot- 

 like mandibles are hardened and calcareous, and are well 

 adapted to crush the hard shells of crabs and other Crus- 

 tacea. A few specimens only have been taken in a perfect 

 state. 



Sjoecies of Nautilv^. 



ambiguus, Soiv. Pompilius, Linn. 



macromphalus, Sow. stenomphalus, Sow. 



perforata SjCow/'acZ. umbilicatus, Lister. 



