56 CRUSTACEA. 



often disproportion ably smaller than the others, and it is this pair 

 which takes the swimming character in the natatory species this 

 form being of a lower grade embryologically, than the gressorial. In a 

 grade below the Macroura, the Anomobranchiates, these same legs are 

 again found to fail of development. 



It is a fact worthy of notice, that the genus Dromia, which is 

 higher than Latreillia or Homola, in having fossettes to the inner 

 antennae, has a mark of a lower character in having two pairs of legs 

 dorsal and abbreviated instead of one. But may not the condition 

 of the cephalic functions have a relation to the latter? May not the 

 cephalic progress towards the Brachyural type, show in what way or 

 degree the forces were exerted in the different directions in these dif- 

 ferent Anomoural forms ? 



Taking the space between the mandibles and the first pair of an- 

 tennae, as the region which may be called the centre of development, 

 since this part first appears in the progress within the egg, and look- 

 ing upon the succeeding developments as going on anteriorly and 

 posteriorly in the cephalothorax from this point as a centre, the 

 actual distance to which such a development of members goes on, may 

 be regarded as inversely as the force required for them. The greater 

 the force required, the less the distance.* The large amount of force 

 required for the cephalic organs (the senses), is thus indicated by the 

 shortness of the distance, and the more perfect the concentration or 

 close conjunction of these members (which is equivalent to the 

 shorter the space, provided the results are well perfected), the higher 

 the grade of the species, and the greater the concentration of force 

 expended in the result. The formation of the cephalic organs pre- 

 cedes that of the thoracic legs, yet not their completion. They 

 have an earlier existence, but their perfection goes on in continued 

 progress as with higher orders of animals, and they are not ordinarily 

 finished until the whole form is complete. Hence, while the suc- 

 cessive developments are going on posterior to the mandibles, there is 

 successive progress anterior to them, and the centre still holds its first 

 position. When then, as in Latreillia, we find the cephalic organs 

 disjoined or separated, we observe in this fact, evidence of that same 



* That is, the distance to which the development of members goes on, and not mere 

 development of the shell, as in the growth of the beak, or some analogous enlargement of 

 vegetative character. 



