( 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 study so promptly and so 
generously; since while the species are comparatively very few, they pre 
sent many and extreme diversities of form 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 Asellus or Crangonyx 
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. Rut 
intelligent measures for the increase and preservation of our edible fishes 
*This principle, that structural characters diminish in importance downward , has 
been ignored, I think, by some of our recent ichthyologists. 
