12 J. D. Dana on the Classification of Crustacea. 
Taking this as the true expression of the mean normal size c 
the type, the three primary types will vary in this respect as 24 
(two inches) : 6 : 1. 
''he size 1n 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 cephahie part includes six out of the fourteen cepha- 
lothoracie annuli. In the Daphnioids and the Caligoids, they 
include only five. ln Limulus, only the first four can properly 
be counted as of the cephalie series. In many other Entomos- 
iraca, the mouth organs are nearly as perfect legs as in Limulus, 
and the species although evidently of à low grade, cannot properly 
be removed from the group. limulus has its nearest ally in 
A pus, although this genus has the mouth organs of a Daphnia. 
The lowest species of the type are the Lernzoids. 
A. fourth primary type includes the Cirripeds. It 1s 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. Butif 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. 'lhe body, when it commencoes a se- 
dentary life, increases in magnitude far beyond the COypris or 
Daphnia size; and there is a corresponding loss of power. 'lhe 
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. 'lhusit is with the Medusse.  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, orlose almost every attribute 
of hfe. '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. Itis manifest, 
that the nervous system, or motive power of the female, is abso- 
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