J* D. Dana on the Classification of Crustacea. 27 



structures 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. 



II. 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 (Mysidas, 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 typical size. 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 Calanidas, apparently the highest group, include spe- 

 cies 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 Lernseoids, and the Cirripeds, 

 outgrows vegetatively the proper sphere of action of the system 

 of force within. This result is especially found in sedentary 

 species, as we have exemplified in our remarks on the Cirripeds. 



IY. Size is, 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. The Decapod, as the size diminishes, reaches 

 the lowest limit ; and then, to continue the range of size in 

 species, another structure, the Tetradecapodan, is instituted; 

 and as this last has also its limit, the Entomostracan is intro- 

 duced to continue the gradation ; and, as these end, the Kota- 

 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-fiftieth 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. 



