330 
ATYOIDA POTIMIRIM, A MTJD-EATING FRESH- 
WATER PRAWN. 
By Dr. FRITZ MULLER.* 
T HE cleaning of the branchial cavities is performed in some 
Prawns, such as Palcemon , by the first pair of thoracic 
feet ; in the Hermit Crabs, Porceilunce , &c., by the last pair of 
those feet ; and in the true Crabs, by the flagella of the three 
pairs of jaw-feet. This function is performed in another and 
quite different manner in a small Prawn found in the river 
Itajahy, Atyoida Potimirim. This little Prawn further presents 
so many remarkable peculiarities, that some account of it may 
not be uninteresting even to those readers who do not know 
much about Crustacea. 
The first thing that strikes one is the structure of the hands 
(claws) or chelce , with which, as in the great majority of the 
Prawns, the first two pairs of thoracic limbs are armed. The 
claws, or hands, of Crabs and Lobsters are produced from ordinary 
walking limbs, by the development of a process of the last joint 
but one beyond the articulation of the last joint, the last joint 
and this process being then opposed to each other. We thus 
have to distinguish the movable finger, the immovable thumb 
(the process), and the true hand ; the latter, as every one knows 
who has eaten a crab or a lobster, forming the main mass of 
the claw, and enclosing the powerful muscles which move the 
finger. 
In our Atyoida , however, we can hardly speak of a true 
hand in contradistinction to the thumb ; the chela is cleft 
through its whole length, and the joints of the hand and 
finger are at the same level. To add to the extraordinary 
aspect of the claw, the hand is very freely articulated to the 
lower angle of the deeply-notched fore-arm, and the apical 
third of each of its divisions is furnished with a dense coating 
Kosmos , Band ix. p. 117. 
