416 CRUSTACEA. 



from the extreme delicacy of the covering that invests them, they evi- 

 dently present to the surrounding medium a surface of sufficient extent 

 for the purpose of exposing the blood to its action, thus rendering them 

 efficient substitutes for branchiae, while, at the same time, their move- 

 ments ensure a perpetual renovation of the water in contact with them ; 

 so that, as a necessary consequence, the respiratory process will be 

 accomplished with greater completeness in proportion as the exertions 

 of the animal become more vigorous. In the Crustacea, indeed, we have 

 many interesting and beautiful examples of the connexion between the 

 respiratory and locomotive organs. The amount of respiration must 

 necessarily be equivalent to the expenditure of muscular energy ; and a 

 more elegant manner of ensuring an exact correspondence between the 

 one and the other, than that adopted, could scarcely be imagined ; for, 

 by appending the branchiae to the locomotive agents themselves, the 

 more actively the latter are employed, the more freely will the former 

 receive the influences of the aerated water in which they are immersed. 



Fig. 212. 



Daphnia. 



(1068.) In the Squilla, which swims by means of the movements of 

 its broad tail, it is the false feet beneath the abdominal segments that 

 become branchial organs ; and these, being expanded into broad and 

 vascular lamellae, perform the office of gills. In the Squilla, therefore, 

 and similarly-formed genera, the free movement of the tail ensures the 

 full and complete exposure of the respiratory structures to the surround- 

 ing element. 



(1069.) The more minute Branchiopods, or Entomostraca, as they are 

 called by zoologists, offer, in their mode of reproduction, several remark- 

 able variations from what has been described above ; and a brief account 

 of their most interesting peculiarities is therefore still wanting to com- 

 plete this part of our subject. These little creatures, in fact, seem to 

 form a transition between the class we are now considering and the 

 EPIZOA, which many of them resemble so nearly, that they are still 



