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plumose swimming-bristles. In repose, these swimming feet 
are thrown forward and laid between the thoracic limbs. Un- 
like all other prawns that I have seen, in Atyoida the outer 
branch of the first pair of swimming feet is not laid between the 
last feet of the thorax, but outside over them and the entrance 
to the branchial cavity, so that all the water entering here is 
strained through its plumose hairs.* 
The males of our Atyoida are much smaller than the females; 
I never found the former more than 15 millimetres, while the 
latter were as much as 25 millimetres long. The better the 
males are equipped with offensive and defensive weapons, and 
the more violently they struggle for the possession of their 
females, the more do they usually exceed the latter in size. 
This is the case among the prawns of the Itajahy with the male 
of Palcemon jamcdcensis , whose chelae of more than a foot in 
length almost always bear traces of the battles which it has 
fought with its rivals. Where weapons are wanting, the 
female often attains a larger size, which is probably superin- 
duced by the much greater expenditure of material called for 
by the eggs. To give a second example belonging to the series 
of the higher Crustacea, I will refer to the Tatuira {Hippa eme- 
rita), the males of which likewise appear quite dwarfed by the 
side of the females. Just as in Atyoida the males have become 
or remained unarmed by the adaptation of the chelae to the pur- 
poses of mud-eating, in the Hippa, which burrows in the sand, 
this has come about by the conversion of the apical joints of the 
legs into broad shovels, only available for digging. 
The hands of the male Atyoidce seem to be unfitted not only 
for contests with their rivals, but also for seizing and holding 
the females, and it is therefore not surprising that other arrange- 
ments have been developed in them, such as we do not find in 
other prawns whose hands are sufficiently capable of grasping. 
Thus the terminal spine of the posterior jaw-feet, which is 
straight in the female, is converted in the male into a curved 
claw ; and on the inner surface of the tibiae of the third and 
fourth pairs of feet there is a strong denticulated spine, and 
opposite to it numerous wartlike tubercles, which, like the spine, 
are entirely wanting in the female. 
There is yet another remarkable difference between the 
sexes to which I must refer. In the males, the inferior angle 
of the anterior margin of the carapace is rounded, while in the 
females it is produced into an acute tooth (Simpson’s ‘ spina 
pterygostomiana ’). Exceptions to this rule are very rare. The 
armature of the anterior margin of the carapace has hitherto 
* From Miln e-Edwards’ figure of Caridina typas ( Hist . Nat. Crusst. PI. 
xxv. bis, fig. 4), I conclude that in this respect the nearly allied genus Cari- 
dina agrees with Atyoida. 
