CLASSIFICATION OF CRUSTACEA. 14H 



IV. Size is, therefore, an important element in the system of ani- 

 mal structures. As size diminishes, in all departments of animal life, 

 the structure changes. To the human structure there is a limit ; to 

 the quadruped also, beyond which the structure is an impossibility; 

 and so seems 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 introduced to 

 continue the gradation ; and, as these end, the Kotatoria 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 consti- 

 tute overlapping lines, as has been sufficiently shown. 



V. In the opposite extreme of organic beings, the vegetable king- 

 dom, the same principle is illustrated. Plants may be so minute as 

 to have free motion and activity, as in animals. The spores of certain 

 Algae are known to have powers of locomotion, and some so-called 

 Infusoria, are now admitted to belong to the vegetable kingdom. 

 These are examples of locomotive plants. Now, ordinary plants, like 

 Cirripeds, are examples of sedentary species, that have outgrown the 

 limits of activity. The life-system of a plant, is in fact sufficient in 

 power to give locomotion only to the minute plant-individuals alluded 

 to; and infusorial species of plants retain it, as long as they live. 

 But when, as in the Algae,, vegetative growth proceeds in the enlarge- 

 ment of the minute infusorial spore, it immediately outgrows its 

 activity, and becomes a sedentary plant. In most other plants, the 

 seed have never the minute size which admits of motion. 



The mean size of the Entomostracan type was stated to be one line; 

 of the Rotatorial type, one-sixth of a line ; and we may add, that 

 the mean size of the plant type — understanding by this, as in other 

 cases, the mean size admitting of the highest activity — if deduced 

 from the size of plant-infusoria, would be about one-sixtieth of a line. 



We observe, that the smallest size of the perfect Macroura (first 

 type) is very nearly the mean size as to length of the animals of the 

 second type. So also, the smallest size of the perfect animal of the 

 second type (Tetradecapoda) is very nearly the mean size of the most 

 perfect animals of the third type ; and the smallest size of the perfect 

 animal of the third type is nearly the largest size in the fifth type. 



