MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 109 



totally eyeless. la a number of specimens from a well at Normal, Illinois, kindly sent us by Mr. 

 S. A. Forbes, a minute black speck is seen on each side of the bead in the position of the eyes of 

 Asellus, just above the posterior end of the base of the mandibles. In some specimens these black 

 dots are not to be seen ; in others they are visible, but varible in degree of distinctness. In twelve 

 specimens which I collected in Shaler's Brook in Mammoth Cave I could detect no traces of eyes, 

 and infer that most, if not all, the Mammoth Cave specimens are totally eyeless. It thus appears 

 that different individuals have eyes either quite obsolete if living in caves in total darkness, or, 

 if living in wells, with eyes in different degrees of development up to a certain stage — that repre- 

 sented by black dots — which, however, are so easily overlooked, that we confess, after handling 

 dozens of specimens, we did not suspect that the rudimentary eyes existed until our attention 

 was called to them by Dr. C. O. Whitman when he sent the slides. The European Asellus forelii 

 is also said to be blind. The specimens we received through the kindness of Professor Forel, 

 'which were, unfortunately, dried and spoiled, seemed to be entirely eyeless, though special search 

 was not made for the eye-specks. 



It will be seen that the eyeless Csecidotsea differs from Asellus as regards its brain and organs 

 of sight, in the complete loss of the optic ganglion, the optic nerve, and the almost and sometimes 

 quite total loss of the pigment-cells and lenses. 



After a pretty careful study of numerous vertical sections of the brain of Ccecidotcea stygia as 

 compared with that of Asellus communis, we do not see that there are any essential differences, 

 except in the absence of the optic ganglia and nerves. The proportions of the procerebral lobes, 

 of the ganglion cells, their number and distribution, the size of the transverse and longitudinal 

 commissures are the same. The head and brain as represented is smaller than in Asellus, the 

 form itself being considerably smaller. 



Careful examination of the sections passiug behind the procerebral lobes and oesophageal 

 commissures failed to show any traces of the optic ganglion of either division, or of the ganglion 

 cells and myeloid substance composing it. Every part connected with the optic ganglia seems to 

 be totally abolished. The same may be said of the optic nerve throughout its length. The entire 

 nervous portion of the optical organs are wanting. 



With the eye itself it is different. The modification resulting from a life in total darkness 

 has left traces of the eye, telling the story of degeneration and loss of the organs of sight, until 

 but the merest rudiments of the eye remain as landmarks pointing to the downward path in deg- 

 radation and ruin taken by the organs of vision; the result of a transfer to a life in total or 

 partial darkness, as the case may have been, in the well-inhabiting or cave-dwelling individuals. 



PL XXIII, fig. lb, represents a section through the head of Ccecidotcea stygia behiud the pro- 

 cerebral lobes and oesophageal commissures, showing the absence of any traces of the optic 

 ganglia or optic nerves, but indicating the rudiments of the eye, showing that the pigment mass 

 of the retina and the lenses exist in a very rudimentary condition, while the optic nerve and 

 ganglion are entirely aborted. 



Figs. 1 c and 1 (7-1 d* represent enlarged views of the rudimentary eye of two different speci- 

 mens of C. stygia from Mammoth Cave. In the sections represented by Fig. 1 c, a, b, we see that the 

 number of facets has been reduced apparently to two (o), the rudimentary lenses being enveloped 

 by a black pigment mass. This section, examined by Tolles's £ A, is magnified and drawn to 

 exactly the same scale as that of the eye of Asellus represented by Fig. 1 a. In that figure may 

 be seen the normal size of the lenses and of the retina cells. It will be seen that in Crecidotaja 

 the retina cells are broken down and have disappeared as such, and that the rudimentary lens (or 

 the hyaline portion we suppose to be such) which the retinal pigment incloses is many times 

 smaller than in the normal eye of Asellus. 



On comparing the eyes of the two specimens as shown in Figs, lc and 1 d'-ld'", it will be 

 seen that the eyes in one are considerably larger than in the other specimen. Fig. 1 d" shows that in 

 the eye of this individual there were at least four lenses, if not more, not included in the section. 

 At the point indicated by 1 d, on the edge of the eye, one lens is indicated (though the divisions 

 are wanting), not wholly concealed by the pigment of the retina; a more magnified view is seen 



