RESPIRATION OF CRUSTACEA. 



241 



the carapace ; the other, destined for the exit of the water, is 

 placed at the sides of the mouth (Fig. 575). The renewal of 

 the water in contact with the gills, is effected by the movements 

 of a large valve, situated near this last opening, and formed by 

 lamellated appendages of the second pair of jaws (Fig. 575, z). 

 In other Crustacea, the Scilla for instance, the gills are formed 

 of feathery tufts ; and instead of being inclosed in the thorax, 



FIG. 576. SCILLA ; y, eyes; a, antennae; p', first pair of legs; 

 p*, second pair of legs ; p 3 , three last pairs of thoracic legs : pa, 

 abdominal pro-legs ; b, gills ; g, fin-like members. 



they float freely on the exterior, and are fixed to the abdominal 

 members. In others, as the Sandhoppers, the gills are replaced 

 by membranous vesicles fixed at the base of the legs, beneath 

 the thorax. Lastly, in the Isopod Crustacea, the respiration 

 is effected by means of false abdominal legs, which present a 

 leaf-like and membranous aspect. 



854. A small number of these animals are formed to exist 

 in air ; but they constitute an exception to what has been said, 

 relative to the difference in the structure of the breathing appa- 

 ratus of terrestrial and aquatic animals : for, instead of being pro- 

 vided with lungs or tracheae, they respire by gills like the former ; 

 but these organs are placed in such a manner, as to be surrounded 

 by that degree of moisture, which is necessary for the exercise of 

 their functions. The Land- Crabs^ which are met with in different 

 parts of the globe, but which chiefly abound in the Antilles, 

 offer a remarkable example of this anomaly. Instead of living 

 in the water like ordinary Crustacea, these animals are terrestrial ; 



