THE THORACOSTRACA. 177 



numerous families and species of the entomostraca and branchio- 

 poda, their next superiors in rank, are generally well provided 

 with organs of vision. 



Some of these minute animals, which are generally active in 

 their movements, have no special respiratory organs, the whole' 

 tegumentary surface being made to supply their place ; while in 

 others the foliaceous legs perform the office of gills, the skin or 

 covering of these limbs being so delicate that it admits the 

 vessels which ramify over it to have sufficient contact with the 

 water to allow of the perfect aeration of the blood. Thus these 

 little creatures may be said to breathe through their legs and 

 arms, which may be seen in constant motion playing through the 

 fluid, and causing a constant flow of new particles to the exposed 

 surface of the bloodvessels. 



On a higher stage of crustacean life we find the Edriophthal- 

 mia, whose thorax and abdomen are distinctly composed of 

 articulated segments, or rings, which is not the case in the 

 above-mentioned orders. Here the legs no longer serve as 

 gills, but for the ordinary purposes of locomotion ; certain 

 portions of the extremities, however, are modified in their 

 structure so as to perform the functions of respiratory organs. 

 To this order belong, among others, the nimble Sand- 

 hoppers, who, when disturbed in their privacy, 

 bound into the air by thousands from the wet 

 sea-sand ; the Cyami, or Whale-lice, which fre- 

 quently feed by thousands on the skin of the 

 huge cetaceans ; and the terrestrial Onisci, or 

 Wood-lice, which, generally leading a life of sandnopper. 

 concealment under stones, in walls, in cellars, or in the cavities 

 of decaying trees, come forth from their retreat only in wet and 

 moist weather. 



Thus, rising step by step, we come at length to the most per- 

 fect type of crustacean life to the Thoracostraca, whose eyes 

 are moveable and fixed on stalks, whose head and thorax are 

 covered with a carapace or shield, and whose abdomen only re- 

 tains the annular structure. 



This highest order of the crustaceans is again subdivided into 

 the Stomatopoda, with external branchiae and feet approaching 

 the mouth ; and the Decapoda, which are either long-tailed, like 

 the shrimp or lobster, or short-tailed, like the crab. In the 



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