J. D. Dana on the Classification of Crustacea. 15 
struetures developed, the lower such as is fitted for structures of 
inferior grade and size. In other words, the life-system is of 
different orders for the different types, and the structures formed 
exhibit the extent of their spheres of action, being such as are 
adapted to use the force most effectively, in accordance with the 
end of the species. 
IL. In a given type, as the first, for example, the same system 
may be of different dimensions, adapted to structures of different 
sizes. But the size in either direction for structures of efficient 
action is limited. "To pass these limits, a life-system of another 
order is required. 'The Macroura, as they diminish in size, 
finally pass this limit, and the organisms (Mysids, for example) 
are no longer perfect in their members; an obsolescence of some 
parts begins to take place, and species of this small size are 
actually complete only when provided with the structure of a 
Tetradecapod. 
The extreme size of structure admitting of the highest efficient 
activity is generally three to six times lineally the average or 
mean typiealsize. Of these gigantic species, three or four times 
longer than the mean type, there are examples among the 
Brachyura and Macroura, which have all the highest attributes 
of the species. "There are also Amphipoda and Isopoda three 
inches in length, with full vigorous powers. Among Entomos- 
traca, the Calanidz, apparently the highest group, include spe- 
cles that are three lines long, or three times the length of the 
mean type. 
III. But the limit of efficient activity may be passed; and 
when so it is attended with a loss of active powers. "The struc- 
ture, as in the female Bopyrus and Lernzoids, and the Cirripeds, 
outerows vegetatively the proper sphere of action of the system 
of force within. 'l'his result is especially found in sedentary 
species, as we have exemplified in our remarks on the Cirripeds. 
IV. Sizeis, therefore, an important element in the system of 
animal structures. As size diminishes, in all departments of 
animal life the structure changes. 'To the human structure 
there is a limit; to the quadrupeds also, beyond which the struc- 
ture is an impossibility; and the same seems to be the case 
among Crustacea. "l'he Decapod, as the size diminishes, reaches 
the lowest limit; and then, to continue the range of size in 
species, another structure, the "letradecapodan, is instituted ; 
and as this last has also its limit, the Entomostracan 1s intro- 
duced to continue the gradation; and, as these end, the Rota- 
toria begin. "Thus Crustacea are made to embrace species, from 
a length of nearly two feet (or two hundred and fifty lines) to 
that of a one-hundred-and-fifüeth of a line. "These several types 
of structure among Crustacea do not graduate, as regards size, 
directly from one to another, but they constitute overlapping 
lines, as has been sufficiently shown. 
