CLASSIFICATION OF CRUSTACEA. 1407 



racic annuli. In the Daphnioids and the Caligoids, they include only 

 five. In Limulus, only the first four can properly be counted as of 

 the cephalic series. In many other Entomostraca, the mouth organs 

 are nearly as perfect legs as in Limulus, and the species, although 

 evidently of a low grade, cannot properly be removed from the group 

 Limulus has its nearest ally in Apus, although this genus has the 

 mouth organs of a Daphnia. 



The lowest species of the type are the Lernseoids. 



A fourth primary type includes the Cirripeds. It is of the same 

 rank as regards cephalization as the Entomostraca ; yet, it has so 

 many peculiarities of structure, that it should be regarded as a distinct 

 type, rather than a subordinate division of the third type. 



The mean size of the species of this group is much greater than 

 the same among the higher Entomostraca. But if we regard the 

 young in its active Cypris state, and compare it with the correspond- 

 ing condition of species of Cyproids, we shall discover that the species 

 have, in fact, an abnormal growth; a growth which takes place at 

 the expense of the powers of motion or action in the individuals. 

 The body, when it commences a sedentary life, increases in magnitude 

 far beyond the Cypris or Daphnia size ; and there is a corresponding 

 loss of power. The same force will not move a heavy structure, that is 

 sufficient for the tiny model ; and when the model is enlarged without 

 a corresponding increase in the seat of power, sluggish motion is the 

 necessary consequence. Thus it is with the Medusae. Individuals 

 of the minuter species, or the larger species, when in the young state, 

 are gifted with active powers of motion ; the structure conforms to 

 the forces within : but as the species enlarge, i ihey become slow in 

 movement, or lose almost every attribute of life? The same principle 

 is illustrated again in the Bopyri. The male is a small active animal, 

 related to Jsera and Tanais. The female, of sedentary habits, becomes 

 grossly enlarged and corpulent, so as to exceed by twenty-fold lineally 

 the length of the male, and nearly ten thousand times its bulk. It is 

 manifest, that the nervous system, or motive power of the female, is 

 absolutely no greater than that of the male; and consequently, the 

 capabilities of locomotion will be ten thousand times less, or the 

 female will move but a ten-thousandth of an inch at the most, while 

 the male is moving one inch, a fact with regard to them, as any one 

 is aware of who has seen the incapability of the female to make any 



