'202 CRUSTACEA. 



organs of vision are present in all at some period of 

 their existence, and in the majority of species are of 

 a very complex structure. We find in some both 

 simple and compound eyes, similar in principle to 

 those of insects; both of these forms occur in the 

 king-crab {Liriiulus), and there are eyes of an in- 

 termediate character, such as that of the water-flea 

 (DapJmia), where several clustered lenses and eye- 

 cells are covered by a single smooth and transparent 

 cornea. But in the higher forms of Crustacea, the 

 true compound or faceted eye only, is met with. 

 The facets are not always six-sided, as in insects, but 

 are sometimes square, as in the cray-fish {Astacus 

 fluviatih's). Sometimes the eyes are immoveable ; but 

 in many species they are placed at the end of jointed 

 foot-stalks, of various length, capable of being pointed 

 in different directions; and we often find, in con- 

 nection with these stalked eyes, a furrow, in which 

 they can be laid flat, and thus protected from injury. 

 The organ of hearing is a cavity closed by a delicate 

 membrane, situated at the base of the second pair of 

 antennae, in the lobster and similar forms. In the 

 crabs this is replaced by a small, moveable, shelly 

 disk, pierced with a hole, over which an elastic 

 membrane is stretched. A cavity filled with fluid in 

 each case conveys the vibrations of sound to the 

 proper nerves. 



There can be little doubt that the higher Crustacea 

 are guided to their food by a sense analogous to that 

 of smell ; but where its seat is placed, zoologists 

 have not determined ; the probability is, that it may 

 be connected with the first pair of antennse. Similar 

 observations may be applied to the perceptions of 

 taste ; the sense doubtless exists, and its organ is 

 supposed to be the delicate membrane that lines the 

 mouth and the throat. It is commonly considered 

 that the sensations of touch can be but very feebly, 

 if at all, conveyed through the hard, calcareous 

 crust with which these animals are clothed ; and 

 that this sense can hardly exist, except in those parts 



