248 SUPPLEMENT 



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 

 Avith 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 head. 



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- 

 tennae, it consists, in Astacus and Squillge, 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, 



