MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. Ill 



awaken our interest regarding the question whether other structures of the body may not have a 

 greater development in place of the atrophied eyes. 



" In the blind crayfish of Mammoth Cave the not very high eye-capsules are almost wholly con- 

 cealed under the rostrum, and are of the same white appearance as the rest of the animal. Where 

 we would expect to find a facetted cornea, on the end of the eye-capsule, the skin is of the usual 

 appearance. It consists of a laminated cuticula provided with pore canals and smooth exter- 

 nally. In the mass which fills the interior of the eye-capsule is distinguished the indistinct cellular 

 matrix of the skin; then a delicately fibrous connective tissue, besides a cellular nucleated part, 

 perhaps the remains of the optic ganglion; also transverse muscular fibers; finally, clumps of a 

 darker appearance; indeed the same structures which appear as whitish bodies through the eye- 

 capsule before it is opened. They may possibly be groups of dermal glands. They have a certain 

 streaked appearance, which in part seems to point to a segregation of the plasma in the rods, and 

 might also in part originate from the ruffling and splitting of a homogeneous membrane. Of 

 crystalline lenses and eye pigment no trace was to be discovered. Should we compare with these 

 results those obtained from an examination of Gammarus puteanus, the relations are similar. But 

 in the blind Gainmaridce, which were investigated under better circumstances, there may be seen 

 an optic ganglion, which curves over towards that place in the body where the eye is situated in 

 species provided with organs of sight. A crystalline lens and layer of a dark pigment are also 

 absent in Gammarus puteanus. It is not otherwise with Asellus cavaticus, in which in the region 

 of the eyes neither crystalline lenses and pigment nor corneal borders are present. And of like 

 kind are also the structures in Cambarus pellucidus, which likewise show that all the elements once 

 composing an eye have become atrophied. Only the ganglion may possibly have persisted and 

 may be detected in fresh specimens. The presence of striated muscles suggests the articulation of 

 an eye-stalk." 



THE EYE OP CAMBARUS HAMULATUS. 

 (PI. XXI, figs. 3, 3a-3g.) 



In shape and size the eyes of this species are closely similar to those of Cambarus pellucidus, 

 and the figure of the latter will well serve to represent the shape of the eyes of the species under 

 consideration. The total length of the eye itself without the short stalk is equal to half the breadth 

 of the rostrum of the same specimen at its base. As in the Mammoth Cave form (Eig. 2, c), the 

 cornea is entirely obsolete. Thin longitudinal sections of the eye which have been preserved for 

 several years in alcohol showed that the facets, cones, or crystalline lenses, the retinal rods, with 

 the retina or black pigment layers, were, like the cornea, entirely wanting. Figs. 3-3 6 show that 

 only the optic ganglion with the optic nerve have survived the effects produced by the absence 

 of light. 



Fig. 3 is a longitudinal section through one side of the eye, passing through the optic ganglion 

 (op. gang.), and through the origin of the ocular muscle; Fig. 3 a passes through nearer the middle of 

 the eye, while Fig. 6 b represents a median section through the eye. It will be seen that the epider- 

 mis ;tt the end of the eye, where the cornea is situated, in normal-eyed Cambari (Fig. 1), has the 

 same lamellated structure as in other parts at the side or base of the eye. Along the side the epi- 

 dermis is made up of about ten layers, which are seen to split off in portions of the sections. These 

 layers partly disappear and partly continue, but thin out as they reach the thinner portion of the 

 end of the eye ; but there are no signs of facets or of cones in the integument. 



The hypodermis consists of a single layer of cells (Fig. 3/) whose walls are broken down, 

 seldom visible, while their nuclei are usually separate, rarely crowded ; they are spherical, or oval, 

 with the nucleolus consisting of numerous granules. In the angle made by the beginning of 

 the stalk the hypodermal cells become elongated or fusiform, and are more distinct than elsewhere. 

 It should be noticed that the hypodermis at the end of the eye in the corneal region of normal 

 species is one-layered; or, if on the sides, two or three-layered, as in the hypodermis of the side of 

 the eye as seen in the same section (Fig. 3). 



The space between the hypodermis and the optic ganglion is filled by a loose connective tissue. 

 Next to the hypodermis is a thick layer of dense, clear, structureless tissue, without nuclei. 



