PODOPHTHALMIA. 57 



diminution of force which is exhibited by the partial development of 

 the posterior pair of legs. The force is exerted in an animal modelled 

 after the Brachyural type; and a failure or decrease of energy in one 

 direction has its almost necessary parallel in diminished energy at the 

 other extremity; the first antennae without fossettes, and the second 

 antennas behind the eyes, balance the abbreviated posterior pair of 

 legs. It is not until the grade of the senses is of a lower order, re- 

 quiring less force for development, and a type is assumed which has 

 the anterior or cephalic part more prolonged, that we find again the 

 posterior legs fully formed; and this type is the Macroural. In the 

 Macroura, as in the Brachyura, the forces are well balanced, and every 

 part has its full development; they represent, as we have said, two dis- 

 tinct styles of structure, and the only two among the species under 

 discussion : the Anomoura are a transition grade, or a mixed style. 



In Dromia, the broader front and more distant eyes indicate, as in the 

 Parthenopinea, some degradation from the high Maia rank. The 

 senses are of a lower order, and their development, hence, requires 

 less force; and consequently, although the antennae, as regards the 

 fossettes and relative position, are more as in the perfect type, this 

 alone is not a mark of superiority. There is a sluggishness in the 

 animal that is proof of the low condition of the senses. The progress 

 of growth anterior to the centre of development, for these reasons, 

 may actually require less force, as compared with that required poste- 

 riorly to finish out the full Brachyura, than in Latreillia; and hence 

 the two posterior pairs of legs in Dromia are abbreviated. The 

 abbreviation of the carapax behind in most Anomoura, is another 

 mark of the same general principle — not a necessary though common 

 fact. 



It may be said, that the object of the shorter hind legs of the 

 Dromia and allied species is sufficiently shown in the uses to which 

 they are put, — their enabling the animal to cover its back with shells 

 or foreign substances, and that we need not look to any principle like 

 that here brought forward. But this resource is necessary to the 

 animal only because of its inferior character. They have not the 

 agility, or strength, or wit of the true Brachyura ; and hence take the 

 clumsy aid of some foreign body or material, for self-defence. They 

 have an order of senses or a nervous force approaching that of the 

 Macroura, but placed in a system, that of the Brachyura, which is 

 wielded with vigour only when nerved after the full Brachyural mode. 



15 



