MALACOSTRACA WITH SESSILE EYES. 
203 
lations, of which generally the first seven have each one pair 
of feet, and the following, and last, to the number of seven at 
most, form a sort of tail, terminated by fins, or appendages in 
the shape of stylets. The head presents four antennse, of 
which the two middle are superior ; two eyes and a mouth 
composed of two mandibles, of a tongue, of two pairs of jaws, 
and of a sort of lip, formed by two jaw-feet, corresponding to 
the two upper ones of the decapods. As in the stomapods, 
there exists no flagrum. The last four jaw-feet are trans- 
formed into feet proper, sometimes simple, sometimes termi- 
nating in a forceps, but almost always in a single finger or 
hook. 
According to the observations of MM. Audouin and Milne 
Edwards, the two ganglionic cords of the spinal marrow 
should be perfectly symmetrical, and distinct in their whole 
length ; and according to the observations of M. le Baron 
Cuvier, the onisci would not be remote from them, only that 
these cords do not present in all the segments of the body 
the same uniformity, and have some ganglia less. Thus, ac- 
cording to them, the nervous system of these Crustacea must 
be the most simple of all. In cymothoe and idotea the two 
chains of ganglia would be no longer distinct. Those which 
come immediately after the two cephalic ones would form as 
many small circular masses, situated on the median line of 
the body ; but the cords of communication which serve to 
unite them together to form a continuous chain, would remain 
isolated, and cemented one to the other. It would seem, ac- 
cording to these facts, that these last Crustacea should be, in 
this point of view, more elevated in the animal scale than the 
preceding ; but other considerations appear to us strongly to 
separate the talitri from the onisci, and to place in an inter- 
mediate rank cymothoe and idotea. 
The sexual organs are situated interiorly towards the origin 
of the tail. The first two appendages with which it is fur- 
