CEPHALOPODA 603 



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

 adheres to the shell; it cannot therefore be associated with the re- 

 spiratory and locomotory movements. The "head foot" is produced 

 into two circles of arms which are very numerous; they are re- 

 tractile and adhesive but have no suckers. The anterior part of the 

 region where it touches the shell is very much thickened to form the 

 hoody 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 2ir&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 "segmentation" 

 may, however, be secondary. Certainly the absence of a renoperi- 

 cardial 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. 409 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 arms like Octopus (Fig. 411 b). The gentler swimming move- 

 ments 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 external 

 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. 



The Tetrabranchiata are divided into two groups, the nautiloids 

 and the ammonoids. The first of these contains Nautilus and other 



