Prof. J. D. Dana on Cephalization. 197 



sizes. But the size in either direction for structures of efficient action 

 is limited. To pass these Hmits, a life-system of another order is 

 required. The Macroura, as they diminish in size, finally pass this 

 limit, and the organisms (Mysidse, 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 pro- 

 vided 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 Amphij)oda and Isopoda 3 inches in length, with full 

 vigorous powers. Among Entomostraca, the Calanidee, apparently 

 the highest group, include species that are 3 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 structure, as in 

 the female Bopyrus and LernDeoids, 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 exem- 

 plified in our remarks on the Cirripeds. 



"IV. 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 structure is an impossibi- 

 lity ; 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 Tetra- 

 decapodan, is instituted ; and as this last has also its limit, the 

 Entomostracan is introduced to continue the gradation ; and, as these 

 end, the Rotatoria begin. Thus Crustacea are made to embrace 

 species from a length of nearly two feet (or 250 fines) 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 suffi- 

 ciently shown." 



While on this subject of life-batteries, the writer would suggest 

 that the grand dynamical distinction between Mollusks and 

 Articulates may be this : — 



A Mollusk corresponds to a quantity-battery, but one of very 

 weak force ; that is, it is analogous to a galvanic battery of two 

 or three small pairs at the most. This is indicated, (1) by the 

 structure of the species, especially the absence of all articula- 

 tions, the animal (a locomotive digestive system) being, as it 

 were, in one simple bag ; (2) by the number of ganglions, limited 

 to three ; and (3) by the sluggishness of the animal. 



An Articulate, on the contrary, corresponds to an intensity- 



