HALF AN HOUR WITH SHELL-FISH. 187 



it has done its work in this respect, it is forced by 

 muscular action into a siphon, formed by a folding 

 of the external leathery coat, and out again. The 

 water thus forced outwards becomes a means of 

 locomotion, and forces the animal backwards. Hence 

 it is that the cuttle-fish always swims backwards, 

 locomotion and respiration going on at the same 

 time, and by the same means. 



We have referred to the comparatively high posi- 

 tion which the cuttle-fish holds among the mollusca. 

 In many respects, it seems to connect them with the 

 vertebrata, not only in its complex eyes, but in its 

 internal bone, which answers to a vertebrate struc- 

 ture, and in its nervous structure. That part of the 

 latter placed near the head is termed the " cerebral 

 ganglion," and this is protected by a cartilage that 

 seems to foreshadow the skull in the true vertebrate 

 animals. The sexes of the cuttle-fish are distinct, 

 and the means of reproduction very curious. First, 

 one of the arms of the male becomes seemingly 

 stunted, so as to form a bladder-like appearance. 

 This eventually bursts, and then the arm shoots out 

 much longer than the rest, terminating in a peculiar 

 oval plate. It is the latter, and that alone, which 

 seems to have the function of transmitting the 

 sperm-cells to the female cuttle-fish. On looking at 

 the illustration, it will be observed that each arm is 

 covered with suckers. These form most formidable 

 aids to the animal's carnivorous tastes, and when an 

 arm is once applied, and the sucker fixed, it is easier 



