INTERNAL STRUCTURE OF PTERODINA. 6S 



its intensity and obliquity frequently changed, until 

 the best effect is obtained. 



We find, on thus viewing the Pterodina, that it 

 is a complex, highly-organized creature, liaving its 

 body protected by a carapace^ like the shell of a 

 tortoise, but as flexible as a sheet of white gelatine 

 paper, which it resembles in appearance. Round the 

 margin of this carapace are a number of little bosses 

 or dots, which vary in diiferent individuals. The 

 cilia are not disposed, as at first appeared, in two 

 separate and distinct disks, but are continuous, as 

 in the annexed sketch. Down each side are two 

 long muscular bands, distinctly striated, and when 

 they contract, the ciliary apparatus is drawn in. 

 As this contraction takes place, two apparently elastic 

 bands, to which the ciliary lobes are attached, are 

 bent downwards, till they look like the C springs 

 behind a gentleman's carriage; and they regain 

 their former position of slight curvature, when the 

 cilia are again thrust out. 



The gizzard is three-lobed, and curiously grasped 

 by forked expansions of the handles of the hammers. 

 The tail, or tail-foot, can be withdrawn or thrust 

 out at the Avill of the creature ; and when in a good 

 position for observation, a slight additional pressure 

 will keep it so for examination. Delicate niuscidar 

 longitudinal bands, forked towards the end of their 

 course, supply the means of performing some of its 



