MOLLUSKS 



157 



duced to a mere dot and is practically invisiljle, but when the walls 

 expand the cell enlarges to fidly twenty-five times its former area, 

 and the sudden enlargement of thousands of these little pigment 

 cells, or chromatophores, as they are called, produces a flash of color. 

 There are several sets of these chromatopfiores, some rosin colored, 

 others yellow, hlue-green or brown. Each set may expand inde- 

 pendently or in combination with the others and thus a varied play 

 of color is produced. 



In Cephalopods the sexes are separate, the male being often 

 much smaller than the female. In some forms, such as the paper 

 nautilus, one of the 

 arms of tlie male un- 

 dergoes a curious 

 transformation. It 

 develops within a 

 large sac which 

 bursts leaving a part 

 of the sides of the 

 sac still attached to 

 the arm. The male 

 then places a packet 

 of spermatoza upon 

 the arm, and after 

 seizing the female 

 tlie arm breaks off. 

 and becomes at- 

 tached to her body within the mantle cavity, thus conveying the 

 spermatoza for the fertilization of the eggs. The eggs are usually 

 laid enclosed in gelatinous capsules and the development is direct 

 without any free swimming larval stage such as is characteristic of 

 other mollusks. 



A good general account of Cephalo])ods is given by Professor 

 J. S. Kingsley in tlie " Riverside Natural History," Vol. I. 



The ChamlDered Nautilus, f Nautilus pouqnUus, Fig. 11-1). 

 This most interesting creature is found in the western parts of the 

 trojjical Pacific, and in the Indian Ocean. It lives upon the bottom, 

 and is not usually found in water less than 100 feet deep, being 

 most abundant at a deptla of about 1000 feet. Contrary to popular 



Fig. 11./: THE OHAMliEliEU, OB PEAULY NAUTILUS. 

 From tlie Tropical I'acilic. 



