(17 



KEY TO THE SPECIES MENTIONED IN THE PRECEDING 



PAPER. 



The general neglect of our Crustacea by the students of our local natural 

 history, if not a discredit, is at least a misfortune : for no other class of an- 

 imals accessible to the inland student will repay stud}- so promptly and so 

 generously ; since while the species are comparatively very few, they pre 

 sent many and extreme diversities of foimi and structure. The differences 

 between the orders of this class, — between the families^ even, of some of the 

 orders, — are more profound, penetrate farther into the interior of the animal, 

 affecting structures commonly far more stable, than do the differences be- 

 tween the other classes of the sub-kingdom. In the same order hearts may 

 be present or absent, in the same tribe gills may be filamentous or lamellate, 

 in the same genus so complex an organ as the eye may be well-developed or 

 entirely wanting ; and everywhere not external form alone seems plastic, 

 but internal structure also. Indeed, this is but an instance of a more gen- 

 eral truth. In every well founded sub-kingdom the lowest class stands 

 nearest the point of common origin, — illustrates, therefore, most closely by 

 its diversities the first divergencies of the group from which the later groups 

 have sprung In this primeval group structure must have been much more un- 

 stable than in the later higher ones, else the stable structural characters which 

 now distinguish classes could never have arisen ; and in the lowest present 

 class, which has departed least from the condition of this primeval group, 

 this instability of structure may be expected to persist, — structural differ- 

 ences will have less "value" for purposes of classification.* Hence 

 in the study of the few examples of this lowest class of arthropods, 

 we rapidly acquire a more fruitful knowledge of nature's multiform ad- 

 justments, encounter more numerous and suggestive illustrations of her 

 general laws, than by much longer and more elaborate study of the higher 

 groups. For the amateur and the beginner the Crustacea have further a pe- 

 culiar interest from the fact that the transparency of some of the smaller 

 forms makes possible the direct and easy study of the entire living organism- 

 Nothing better could be devised for the luminous demonstration of the lead- 

 ing facts of animal physiology. In a single colorless Asellns or Crangnni/x 

 may be observed at leisure, under a low power of the microscope, the re- 

 spiratory movement, the circulation of the blood, the motions of the heart 

 and the actions of its valves, the contraction and relaxation of muscular 

 fiber, the processes of digestion, as well as the general and minute anatomy 

 of the entire animal. 



The economical interest of the subject should not be overlooked. With 

 the progressive settlement of the country we must look forward to a con- 

 tinuous advance in the price of animal food, and with this advance the ques- 

 tion of our inland fisheries will rise yearly into higher prominence. But 

 intelligent measures for the increase and preservation of our edible fishes 



*This principle, that structural characters dwiintsh in i77iportance doTimivard, has 

 been ignored. I think, by some of our recent ichthyologists. 



