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one-half their height. Their consistence is that of the cornea 
very much softened, and they are easily crushed under a 
feeble pressure. Their surface is perfectly smooth, and does 
not allow us to perceive any indication of adherence. The 
black part, when separated, looks like a heap of little blackish 
brown grains in a filamentous substance. All this assemblage 
is enveloped by a membrane, spheroidal, perfectly transparent, 
applied immediately to the crystallines. The terminal gan- 
glion of the optic nerve presents, like that of the decapod 
Crustacea, a bundle of little nerves, the number of which ap- 
pears equal to that of the crystallines. These crystallines 
being directed all ways, form by their union a composite eye 
nearly similar to that of the insects, and appear to form, each 
with the part of the globe of the eye which is related to it, a 
simple eye, independent of the others. The general spheroidal 
envelope may be considered as being a cornea common to 
all these simple eyes.” M. Straus presumes that each of these 
simple eyes is provided with a retina, or a choroid. 
This same system of organs is found again in lynceus, 
polyphemus and branchipus, but in these last the composite 
eye is pedunculated, and its general cornea is exterior, instead 
of being enclosed in the .Jiead. 
The eyes of many entomostraca are moved by four muscles, 
which carry them in very various directions. 
It is certain that many of the Crustacea possess the sense of 
hearing ; for noise produces an evident impression upon 
them. Nevertheless, it is probable, that this sense is very 
much obliterated in most of the entomostraca. It is only in 
the macrouri that the organ of hearing has been discovered 
with any approximation to certainty. Situated in the testa, 
at the lower part of the first articulation of the external an- 
tenna?, it consists, in Astacus and Squilla?, of a cavity pierced 
in the thickness of this testa, and enclosing a little sac or oval 
vestibule, formed by a slender membrane, of a white colour, 
