NAUTILUS, CUTTLE-FISH, ETC. 



645 



considerable interest. Scattered over the surface of the body, and just 

 beneath the outer layer of the skin, are a number of pigment cells called 

 chromatophores, and it is by the contraction and expansion of these, which 

 are partly under the control of the animal, that the changes of colour are 

 effected. 



The class is divided into two orders, according to the number of gills 

 present, viz.: Tetrabranchiata and Dibranchiata. 



ORDER I. TETRABRA.NCHIATA 



Is represented at the present day by a single genus Nautilus; but in past 

 times there were many other very strange forms whose shells were either 

 uncoiled or curved or quite straight. 



The shell of Nautilus is entirely external and coiled forwards over the 

 animal's back ; the aperture of the shell of the male is proportionately wider 



than that of the females. 

 Its outer layer is porcella- 

 neous, the inner nacreoua, 

 Inside, when neatly cut in 

 half, it is seen to be divided 

 off by a number of shelly 

 partitions into a series of 

 chambers (Fig. 23). The 

 reason for these lies in the 

 fact that the animal does not 

 occupy the hinder part, which 

 it shuts off each time that 

 growth compels it to con- 

 struct a wider and more 

 commodious portion in front. 

 The first chamber of all, or 

 protoconch, being horny, is 

 never preserved in the Nau- 

 tiloidea. Running right 

 through these chambers to 

 the last one is a small tube, 

 the walls of which are pervious and lined with a black horny layer. It is 

 occupied by a backward extension of the mantle called the siphunde. What 

 the function of this siphuncle may be is still unknown, nor is it yet 

 ascertained for certain whether the untenanttd chambers are filled wkh gas 

 or water, though the latter appears more probable. 



The animal of Nautilus is rare, and differs conspicuously from other 

 Cephalopods in that it has no long arms furnished with suckers; the mouth 

 is, instead, surrounded by numerous suckerless tentacles which can be 

 retracted into special sheaths. The margins of the lobes which form the 

 funnel are not united. There is no ink-bag. The gills are four in number. 



The eye is remarkable for being of the simplest description : it consists of 

 a cup-shaped depression lined by the retina and covered with an outer layer 

 of the skin, save for one small aperture in the centre. The sea-water fills the 

 cavity when the animal is alive, and the whole apparatus must work on the 

 principle of a pin-hole camera. 



Four species of Nautilus are known from the Gulf of Persia, the Indian 

 Ocean, the China Sea, and the Pacific Ocean. 



Fig. 23. NAUTILUS. 



