TETRABRANCHIATA. 



827 



foot is thickened, and forms a fleshy fold or "hood" (fig. 738, c), 

 which abuts against the preceding whorl of the shell. On the ven- 

 tral side the median portion of the foot is converted into two mus- 

 cular lobes, which are placed in apposition but are not fused with 

 one another, thus forming the " funnel." As in all the Cephalopods, 

 the funnel serves for the expulsion of the currents of water which 

 have passed over the gills, as also for the escape of the undigested 

 portions of the food and the generative products. 



The mouth is placed in the centre of the head, and is armed with 

 two horny mandibles, partially calcified towards their extremities, 

 and shaped like the beak of a parrot, except that the inferior man- 



Fig- 738. — Pearly Nautilus (Nautilus p07npilius). «, Mantle 

 c, Hood ; o, Eye ; t, Tentacles ; f, Funnel, 



b, Its dorsal fold : 



dible is the longest. A "radula" is present, and the intestine 

 terminates at the base of the funnel. The heart is contained in a 

 large viscero-pericardial sac (or " pericardium "), and consists of a 

 ventricle, which receives four branchial veins ; but there are no 

 " branchial hearts." The branchiae are contained within the mantle- 

 cavity, and are four in number, two on each side. No ink-gland is 

 developed. The sexes are distinct, and the reproductive organs of 

 the female consist of an ovary, with two oviducts (the left rudi- 

 mentary), and an accessory nidamental gland. 



The only structures possessed by the Pearly Nautilus which are 

 capable of preservation in the fossil condition are the mandibles and 

 the shell, since the foot in this genus secretes no structure which 

 can be compared with the " operculum " of the Gastropods. Leav- 

 ing out of sight the " Aptychi " of the Ammonites, it is from fossil 



