CEPHALOPODA 527 



compared with those in Sepia (Fig. 377). The shell coils forward over 

 on the neck of the animal (exogastric) ; the mantle cavity is posterior as 

 in all cephalopods. In other words differential growth of the visceral 

 hump is not here associated with torsion. The mantle is thin and ad- 

 heres to the shell ; it cannot therefore be associated with the respiratory 

 and locomotory movements. The " head foot " is produced into two 

 circles of tentacles which are very numerous ; they are retractile and 

 adhesive but have no suckers. The anterior part of the region where 

 it touches the shell is very much thickened to form the hood, and when 

 the animal is retracted into the living chamber the hood acts as an 

 operculum. The third region of the head foot is the funnel, here 

 composed of two separate lobes. 



The other principal points in which Nautilus differs from the rest 

 of the living cephalopods are as follows : 



(i) There 2iVQ four ctenidia and /owr kidneys, without renopericar- 

 dial apertures. The pericardium opens independently to the exterior 

 by a pair of pores. The fact that in the most primitive cephalopod 

 now existing there is a kind of segmentation of the body cavity 

 and mantle organs has been taken to support the origin of the 

 cephalopods from a metamerically segmented ancestor. This ''seg- 

 mentation" may, however, be secondary. Certainly the absence of 

 a renopericardial connection is not a primitive feature. There is 

 nothing to prove that the fossil chambered-shell cephalopods had 

 four ctenidia and four kidneys. 



(2) There are very simple eyes (Fig. 384 A) consisting of an open 

 pit with no lens, the surface of the retina being bathed by sea water. 

 This appears to be a primitive feature, but Nautilus is nocturnal and 

 the eyes may have undergone reduction. 



(3) There is no ink sac in Nautilus, nor apparently in the other 

 forms grouped in the Tetrabranchiata. 



Nautilus lives at moderate depths on some tropical coasts. It either 

 swims near the bottom or crawls over the rocks, pulling itself along 

 by its tentacles like Octopus (Fig. 387). The gentler swimming 

 movements are caused by the contraction of the muscles of the funnel 

 only ; the more violent movements are probably caused by the animal 

 suddenly withdrawing into the shell, thus expelling the water from the 

 mantle cavity. It is nocturnal and gregarious and a ground feeder. 



The chief interest of Nautilus lies in the fact that it is the sole living 

 representative of a vast multitude of cephalopods with chambered 

 shells which flourished between the earliest Cambrian and the late 

 Cretaceous period, a space of time embracing much the longest part 

 of the history of life on the earth. After being the dominant type of 

 marine invertebrate in the Mesozoic they suddenly became extinct, 

 and the Cephalopoda are now mainly represented by the Dibranchiata 

 with their internal shells. 



