ON CRUSTACEA. 
247 
has observed in those of Palinurus. According to him, the 
optic nerve traverses the ocular peduncle by a cylindrical 
canal which occupies its axis. Arrived at the centre of the 
convexity of the eye, it forms a little button, from which pro- 
ceed in all directions very fine threads, meeting at some dis- 
tance, the choroid membrane, which is nearly concentric to 
the cornea, and which envelopes this spherical tuft of the 
extremity of the nerve, like a hood. All the distance between 
this choroid, and the cornea, is occupied, as in the insects, 
by compact whitish threads, which go perpendicularly from 
one to the other, and whose extremity, which touches the 
cornea, is equally invested with a black varnish. These 
threads are the continuation of those produced by the button, 
which terminates the optic nerve, and which have pierced the 
choroid. 
The eyes of oniscus, garnmarus , and other isopoda, or 
ampliipoda, have not been examined ; but those of certain 
entomostraca, such as daphnia and branchipus, have. The 
daphnia, in the first moment of their development, appear to 
have two distinct eyes, but when they are more aged, these 
two eyes are confounded into a single one. Swammerdam and 
Leeuwenhoek regard as double the single eye of these animals 
in the adult state, while Geoffroy, Degeer, Jurine, and Straus, 
consider it as simple. “ Placed at the most anterior part of 
the head, ’ says this last naturalist, “ this single eye is covered 
by the general envelope, which communicates no modification 
to this spot. Its form is that of a sphere, moveable on its 
centre in all directions. Its surface is furnished with about 
twenty crystallines ( areoles , Jur.), perfectly limpid, placed at 
small distances one from the other, and rising in a hemisphere 
on a black ground, which forms the mass of the eye, but being 
isolated, these crystallines present themselves under the form 
ol a pear, being in their natural situation encased by their 
lesser extremity in the globe of the eye, as far as beyond 
