308 COMPARATIVE ANATOMY AND PHYSIOLOGY, 



In many cases lines of growth can be made out, 

 and the shell may continue to increase in size for so 

 long a time that a single valve of a Tridacna will be 

 found to weigh a hundred and fifty-six pounds. 



In Nautilus the shell consists of a number of 

 chambers (Fig. 126), each of which is larger than that 

 which precedes it, and is formed by the animal as it 

 increases in size ; each of these chambers is separated 



Fig. 126. A Section of tbe Shell of the Pearly Nautilus, showing the 

 successive Chambers occupied by the Animal. 



a, Mantle; b, dorsal fold; g, muscle; ii, siphuncle; fc, funnel; n, hood;p, ten- 

 tacies ; , eye; x, septa ; z, last chamber. 



from one another by a septum (x), and the whole 

 mass is spirally coiled on itself. The chambers are 

 connected with one another by a tube or siphuncle 



(ii), the presence of which has given rise to the belief 

 that the whole series forms a kind of float by means 

 of which the animal is enabled to remain at will on the 

 surface of the water ; such definite observations, 

 however, as have been made on living specimens, 

 and the fact that though (like the shells of Spirula) 

 the shells are common enough, but the animals very 

 rare, lead us rather to believe that Nautilus is essen- 

 tially a dweller at the bottom of the ocean. Here, 



