ATYOIDA POTIMIRIM, A MUD-EATING FRESHWATER PRAWN. 333 
cleanse them. This is effected by the posterior maxilla). It 
is well known that in all Decapoda these maxilla) have exter- 
nally a large broad plate, which is placed as a valve at the 
outlet of the branchial cavity, and by its movements maintains 
the respiratory current ; in this we distinguish more or less 
distinctly an anterior part and a posterior section, of which 
the former must be interpreted as the outer branch, and the 
latter as the flagellum. When the plate serves only as a valve 
for regulating the respiratory current, this hinder section is 
short and truncated, or rounded off at the end, and scarcely 
extends into the branchial cavity itself ; in Palcemon , for 
example, it reaches only to the gill of the outer jaw-foot. In 
Atyoida , on the contrary, this hinder portion is long and 
narrow, tapering towards the extremity, and here furnished 
with about a dozen very long flexible bristles ; it reaches as 
far as the last gill but two, which is seated upon the third pair 
of feet, and its apical bristles to the posterior extremity of the 
branchial cavity. Thus, as may easily be ascertained by the 
examination of sufficiently transparent living animals, the 
whole outer surface of the branchiae may be swept by it. 
Another arrangement may assist in keeping the branchial 
cavity clear, and this occurs also in many other prawns, such as 
the very large genus Hippolyte. The posterior jaw feet and a 
variable number of the thoracic feet (in Atyoida Potimirim the 
first three pairs) bear a minute flagellar appendage, which, 
from its small size, might be regarded as stunted if its peculiar 
structure did not contradict any such notion. In our Atyoida , 
it may be described as a small sausage-like appendage which 
originates near the anterior margin of the coxa, and, being 
directed backward, is applied by its inner surface to the outer 
surface of the coxa. Its outer surface is beset with about a 
dozen rather long, straight hairs, arranged in two rows, and its 
free extremity is furnished with a hook, which probably serves 
to keep it in position. These flagellar appendages then lie in 
the entrance to the branchial cavity, in the fissure between the 
coxa) of the feet and the lower margin of the carapace ; they 
narrow this entrance, and thus, as also by their armature of 
hairs prevent the entrance of foreign bodies. 
But, it will be objected, they are deficient precisely where 
they would be most necessary, namely, where the water flows 
most rapidly into the branchial cavity, above the fourth and 
fifth pairs of feet. Here, however, there is another very pecu- 
liar arrangement for the same purpose, and one that, so far as I 
know, has been observed in no other prawn. The abdominal 
feet of the prawns, as is well known, are (with few exceptions) 
of two branches ; the branches usually have the form of tongue- 
shaped leaves, the margins of which are fringed with long, 
