24 



/. D. Dana on the Classification of Crustacea. 



Taking this as the true expression of the mean normal size of 

 the type, the three primary types will vary in this respect as 24 

 (two inches) : 6:1. 



The size in this third type, reaches its maximum in the Limuli ; 

 and these are unwieldly species, whose very habits show that 

 vegetative growth has given them a body beyond the successful 

 control of its weak system, that is, a larger frame than it has 

 power to wield with convenience, or defend, for it is at the mercy 

 even of the waves upon a beach. . 



This type has its highest representatives among the Cyclopoids, 

 which remind us of the Mysis group of the higher Crustacea. 

 In these, the cephalic part includes six out of the fourteen cepha- 

 lothoracic 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 Entomos- 

 traca, 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 Lerngeoids. 



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 

 corresponding 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 se- 

 dentary 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 cor- 

 responding increase in the seat of power, sluggish motion is the 

 necessary consequence. Thus it is with the Medusas. Individ- 

 uals of the minuter species, or the larger species when in the 

 young state, are gifted with active powers of motion ; the struc- 

 ture conforms to the forces within: but as the species enlarge, 

 they 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 Jaera 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 abso- 



