48 NATURAL HISTORY OF ARTHROPODS. 



been suspected. These two pairs of feet are closer together than the rest, and the 

 adjoining surfaces of their basal joints are smooth and polished, and their margins bear 

 a dense border of long, silky, and peculiarly formed hairs. Milne Edwards, who com- 

 pares these surfaces as to appearance with articular surfaces, thinks that they serve to 

 diminish the friction between the two feet. In considering this interpretation the 

 question could not but arise, why such an an-angement for the diminution of friction 

 should be necessary in these particular crabs, and between these two feet, leaving out 

 of consideration the fact that the remarkable brushes of hair, which, on the other hand, 

 must increase friction also remain unexplained. But, upon bending the feet of a large 

 sand crab to and fro in various directions, in order to see in what movements of the 

 animal friction occurred, at the place indicated it was noticed, when the feet were 

 stretched widely apart, there was in the hollow between them a round orifice of con- 

 siderable size, through which ak could be blown into the branchial cavity, and a small 

 rod might even be introduced. While in Grapsus the water reaches the branchiae 

 only in front, in Ocypoda it flows in through this orifice. A somewhat similar struc- 

 ture is found in two species of fiddler crabs, and our author is disposed to regard the 

 hau's mentioned as possibly organs of smell. 



It may not be amiss to state in this connection the reasons why most gill-bearing 

 animals die when taken from the water. Although the amount of oxygen present in 

 the air greatly exceeds that in water, the gills, usually soft in character, so mat to- 

 gether when the supporting influence of the water is withdrawn, that the extent of 

 surface available for respiration is not sufficient for the needs of the animal, and hence 

 suffocation ensues. 



The Decapoda are usually divided into Macrura or long-tailed crabs, Brachyura or 

 short-tailed crabs, and a third group, Anomura, standing between the two first men- 

 tioned, and to a certain extent combining the characters of each. When, however, 

 we come to study the embryology it is seen that the members of the Anomura should 

 be divided among the two groups first mentioned ; and further, that the usually 

 adopted arrangement of the divisions of the Brachyura does not represent their true 

 relationships. So, following the hints afforded us by the development, which, when 

 properly interj)reted, are in full accord with those furnished by comparative anatomy, 

 we will divide the Decapoda into two sub-orders, Macrura and Brachym-a, each in turn 

 containing several distinct sub-divisions. 



Sub-Obdee I. — Mackuea. 



The Macrura, embracing the shrimps, prawns, lobsters, crayfish, and hermit crabs, 

 are characterized by the possession of an elongate body, the abdomen being very large 

 and not habitually folded under the thorax. The carapax is frequently long and 

 cylindrical. Both pairs of antennae are long and fihform ; the inner pair are never 

 folded away in little pits, as in the Brachyura ; the outer pair frequently bear a lamellar 

 appendage at the base, the modified exopodite. The buccal area is not margined in 

 front, while the external maxillipeds are strongly pediform. Attached to the abdomen 

 are usually six pairs of appendages, the sixth pair uniting with the last segment of the 

 body to form the powerful caudal fin, so useful to these animals in locomotion. With 

 the exceptions of the Penseidea and Astacidea, whose development will be men- 

 tioned further on, the Macrura hatch from the eggs as zoeas, in which the first eight 

 appendages (ending with the external maxillipeds) are present, while all of the thoracic 



