SENSES. 85 



to use different terms, indicating the precise functions of each 

 organ. 



The class which we are now considering may he regarded 

 as taking the place in the water which insects occupy on 

 land. They are very varied in their forms, as well as size, 

 some of them being in fact the giants of the sub-kingdom to 

 which they belong, whilst others are of a microscopic minute- 

 ness. Some species are of a globular or oval shape, others 

 square, whilst a few are linear and elongated ; some again 

 have the body quite flat, others are compressed, and in a few 

 the covering of the body, instead of being hard and crusta- 

 ceous, exists in a softened and membranaceous state. 



These animals possess the ordinary senses which the infe- 

 rior animals are gifted with, although, from the great modifi- 

 cation in their structure, as compared with that of the verte- 

 brated higher animals, it is difficult, and, indeed, often 

 impossible for us to assign them to their legitimate organs. 

 That they taste is evident from the fact that they are not 

 indifferent to the kind of food which they meet with, and 

 which, indeed, they seek with much assiduity. The very 

 complex organization of their mouths, the developement of 

 which far surpasses that of insects, must doubtless be re- 

 garded as indicating the seat of this sense. The organs of 

 sight are very distinct, in the greater number existing as 

 facetted eyes, borne upon footstalks, often of a very great 

 length, as in Podophthalmus ; often, however, they are ses- 

 sile, that is, not elevated upon the surface of the head or 

 shell. The structure of the eyes of these animals has lately 

 been studied very minutely by M. Muller (Ann. Sc. Nat., 

 July, 1829). The sense of touch, from the hard envelope in 

 which these animals are incased, must in all probability be 

 greatly diminished, especially in the more crustaceous spe- 

 cies. The circumstance that they are provided with two 



I 



