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 sj^herical tuft of the 

 extremity of the nerve, like a hood. All the distance between 

 this choroids and the cornea, is occupied, as in the insects, 

 by compact whitish threads, which go perpendicularly from 

 one to the other, and whose extremity, wdiich 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, gammarus, and other isopoda, or 

 amphipoda, 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. Swammerdara and 

 Leeuwenhoek regard as double the single eye of these animals 

 in the adult state, while GeofFroy, Degeer, .7 urine, 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 

 of a pear, being in their natural situation encased by their 

 lesser extremity in the globe of the eye, as far as beyond 



