292 FUNCTIONS AND INSTINCTS. 



large bands, swimming over the surface by 

 means of a floating apparatus consisting of aerial 

 vesicles, produced by their foot ; and attached 

 to its posterior part, a little below the point to 

 which the operculum is fixed in other genera, and 

 to which Cuvier thinks it bears some analogy, 1 

 who also observes that it has a natatory mem- 

 brane or fin on each side of its body. During 

 this action their head is very prominent, and the 

 foot is so extended that the float or line of 

 vesicles forms an angle with the middle of the 

 shell. When the sea is rough, the animal ab- 

 sorbs the air from its vesicles, changes the direc- 

 tion of its foot, contracts its body, and lets itself 

 sink. It does the same when in danger from 

 any enemy, and further, like the cuttle-fish and 

 some others, colours the water by the emission 

 of a blue fluid, which serves to conceal it. 

 They are vividly phosphoric in the night. Birds 

 carry them off with great dexterity. 



If their floating apparatus is mutilated the 

 foot can reproduce it. The latter is flat towards 

 the head, this part of it is furnished with a 

 transparent membrane, which extends far be- 

 yond its extremity, and is composed of a large 

 number of vesicles of unequal size, those in the 

 middle being the largest ; these vesicles the 

 animals fill with air at their pleasure. The 



1 PLATE VI. FIG. 2. a. 



