CEPHALOPODA. 85 



on their inner surface, and are retractile within sheaths, or " digitations," 

 which correspond to the eight ordinary arms of the cuttle-fishes ; their supe- 

 riority in number being indicative of a lower grade of organization. Besides 

 these there are four ocular tentacles, one behind and one in front of each eye ; 

 they seem to be instruments of sensation, and resemble the tentacles of doris 

 and aplysia (Owen}. On the side of each eye is a hollow plicated process, 

 which is not tentaculiferous. The respiratory funnel is formed by the folding 

 of a very thick muscular lobe, which is prolonged laterally on each side of the 

 head, with its free edge directed backwards, into the branchial cavity ; behind 

 the hood it is directed forwards, forming a lobe which lies against the black- 

 stained spire of the shell (fig. 43 *.)* Inside the funnel is a valve-like fold 

 (fig. 44 s). The margin of the mantle is entire, and extends as far as the 

 edge of the shell ; its substance is firm and muscular, as far back as the line 

 of the shell-muscles and horny girdle, beyond which it is thin and transparent. 

 The shell-muscles are united by a narrow tract, across the hollow occupied by 

 the involute spire of the shell ; and are thus rendered horse-shoe shaped. 

 The siphuncle is vascular ; it opens into the cavity containing the heart ( pe- 

 ricardium}, and is most probably filled with fluid from that cavity. (Owen.) 



Respecting the habits of the nautilus, very little is known, the specimen 

 dissected by Professor Owen had it crop filled with fragments of a small crab, 

 and its mandibles seem well adapted for breaking shells. The statement that 

 it visits the surface of the sea of its own accord, is at present unconfirmed 

 by ob servation, although the air cells would doubtless enable the animal to 

 rise by a very small amount of muscular exertion. 



Professor Owen gives the following passage, from the old Dutch naturalist, 

 Rumphius, who wrote in 1705, an account of the rarities of Amboina. 

 " When the nautilus floats on the water, he puts out his head and all his tenta- 

 cles, and spreads them upon the water, with the poop of the shell above water ; 

 but at the bottom he creeps in the reverse position, with his boat above him, 

 and with his head and tentacles upon the ground, making a tolerably quick 

 progress. He keeps himself chiefly upon the ground, creeping also sometimes 

 into the nets of the fishermen ; but after a storm, as the weather becomes 

 calm, they are seen in troops, floating on the water, being driven up ly the 

 agitation of the waves. This sailing, however, is not of long continuance ; 



* The funnel is considered the homologue of the foot of the gasteropods, by Loven, 

 a conclusion to which we cannot agree. The cephalopoda ought to be compared with 

 the larval gasteropods, in which the foot only serves to support an operculum; or 

 with the floating tribes in which the foot is obsolete, or serves only to secrete a nida- 

 mental raft (ianthina). However, on examining the nautilus preserved in the British 

 Museum, and finding that the funnel was only part of a muscular collar, which ex- 

 tends all round the neck of the animal, we could not avoid noticing its resemblance 

 to the siphonal lappets of paludina, and to that series of lappets (including the oper- 

 culigerous lobe) which surrounds the trochus (fig. 87). 



