1410 CRUSTACEA. 



The third and fourth types show their inferiority to the second, by 

 the absence of a series of abdominal appendages; and the fifth a lower 

 state still, in the absence of both thoracic and abdominal legs. The 

 more degraded Macroura (certain Mysidae) show a transition in this 

 obsolescence of abdominal organs to the third type. 



Some of the conclusions from these facts are the following. 



I. Each type corresponds to a certain system of force, more or less 

 centralized in the organism, and is an expression of that force, the 

 higher degree being such as is fitted for the higher structures deve- 

 loped, 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 dif- 

 ferent 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 re- 

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

 and the organisms (Mysidae, 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 

 Amphipoda and Isopoda three inches in length, with full vigorous 

 powers. Among Entomostraca, the Calanidse, apparently the highest 

 group, include species 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 structure, as in the 

 female Bopyrus and Lerngeoids, and the Cirripeds, outgrows vegeta- 

 tively 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. 



