ORGANS OF THE SENSES. 



and surrounded by the pigmentum and the choroid, excepting 

 on the fore part, where it bounds the pupil like an iris, and 

 on the back part, where it is penetrated by the optic nerve. 

 The optic nerve expands into a cup-like retina investing, with 

 the hyaloid membrane, all the convex posterior surface of the 

 vitreous humour. A disposition of the vitreous humour 

 with its retinal and hyaloid membranes, very similar to this, 

 has been observed even in the compound eyes of some noc- 

 turnal lepidopterous insects, which is perhaps more general 

 in that class. So that this optical instrument has already all the 

 essential parts presented by the highest forms of the organ, and 

 approaches the nearest to that of the vertebrata. The structure 

 of the simple eyes or ocelli of the mygaleand of the tarantulse 

 appears to be the same as that of the scorpion. The eyes of 

 crustaceous animals are compound, like those of insects ; 

 in the higher orders they are pedunculated, and moveable 

 by means of muscles inserted within their exterior hard 

 sclerotic covering ; they are commonly sessile and immove- 

 able, like those of insects, in the inferior Crustacea; and in 

 the lowest entomostracous forms the two sessile eyes are 

 frequently united on the median plain, to form a single 

 organ, a character approximating these parasitic Crustacea to 

 the epizoa and to many other inferior articulata. The 

 internal structure of these compound eyes is nearly the same 

 as those of insects, and was early illustrated by Lewenhoek, 

 who first observed the numerous small conical crystalline 

 lenses within the exterior layer of contiguous prismatic 

 transparent cornese in the astacus fluviatilis. The epidermis 

 in the compound eyes of Crustacea passes transparent and 

 homogeneous over the exterior surface of the thick layer of 

 prismatic cornese, which are here, as in insects, generally 

 hexagonal, but sometimes quadrangular, and to the internal 

 ends of the prismatic cornese are applied the broad bases of 

 the hard tapering transparent lenses which have their internal 

 truncated apices directed to the retinal expansions of the 

 numerous optic nerves. The whole sides of these transparent 

 conieal lenses, as well as the optic nerves extending back- 

 wards from their inner ends, are covered, as in insects, 

 with the dark choroid pigment, so that only a small pencil of 

 light gains admission through these long narrow darkened 

 tubes to the small aperture at the posterior truncated ends 



PART III. S 



