1 10 MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 



at Fig. 1 d*. The four sections 1 d' to 1 d<" passed through the eye, the sections in front and behind 

 not touching the eye itself. 



It thus appears from the observations here presented that the brain of the blind Caecidota3a 

 differs from that of the normal Asellus in the absence of the optic ganglia (both divisions) and 

 the optic nerves, while the eyes are exceedingly rudimentary, the retinal cells being wanting; the 

 black pigment mass inclosing very rudimentary minute lens-cells, which have lost their transverse 

 zonular constriction or division; the entire eye of Csecidotaea finally being sometimes wanting, 

 but usually microscopic in size, and about oue-fifth as large as that of the normal Asellus. 



The steps taken in the degeneration or degradation of the eye, the result of the life in dark- 

 ness, seems to be these: (1) the total and nearly or quite simultaneous loss by disuse of the optic 

 ganglia and nerves; (2) the breaking down of the retinal cells; (3) the last step being, as seen in 

 the totally eyeless form, the loss of the lens and pigment. 



That these modifications in the eye of the Crecidotaea are the result of disuse from the absence" 

 of light seems well proved ; and ihis, with many parallel facts in the structure of other cave 

 Crustacea, as well as iusects, arachnids, and worms, seems to us to be due to the action of two 

 factors: (a) change in the environment; (b) heredity. Thus we are led by a study of these 

 instances, in a sphere where there is little if any occasion for struggling for existence between 

 these organisms, to a modified modern form of Lamarckianism to account for the origination of 

 these forms, rather than to the theory of natural selection or pure Darwinism as such. 



The brain of Crangonyx. — Sections were made of two species; one from Illinois with eyes (the 

 species not identified), besides sections of the eyeless G. vitreus, from Mammoth Cave. In the 

 Illinois Crangonyx the brain, as seen in section, is represented by PI. XXYI, fig. 1. The eye was 

 torn out in making the sections, so that the actual position of the eye and the relations to it of the 

 optic lobes was not satisfactorily determined. Figs. 2 to 7 represent sections of a large specimen 

 from in front backwards. In Fig. 7 are seen the relations of the optic ganglia and olfactory ganglia 

 to each other. 



When we compare with these Figs. 8 to 12 of Crangonyx from Mammoth Cave, especially com- 

 paring the sections represented by Figs. 7 and 10 a, we see very slight differences between the 

 brains of the eyed and the eyeless Crangonyx. The optic ganglia have about the same proportions 

 as do the other lobes and the arrangement of the ganglion cells. Perhaps striking differences 

 should not be expected, as the eyes of the eyed species of Crangonyx are small compared with 

 those of Gammarus. 



STRUCTURE OP THE BRAIN AND RUDIMENTARY EYES OE BLIND CRAYFISH. 



The first author to describe and figure the eyes of Orconectes psllucidus of Mammoth Cave was 

 George Newport, in his memoir " On the Ocelli in the genus Anthophorabia," Trans Linn. Soc. 

 (Tab. X, figs. 11-14). While the external form of the eye is rendered correctly, this usually accu- 

 rate and painstaking anatomist makes some strange errors both in his description and figures of 

 the internal structure of the eye. The eye itself, he correctly states, is destitute of a pigmentary 

 choroid, but then he remarks that " the hardened tegument which clothes the entire organ is 

 thinnest and most transparent in that part which forms the cornea (b) in other crustaceans; so that 

 although the eye may be unfitted for distinguishing form, the creature may yet possess the faculty 

 of perceiving the small amount of actinic rays of light which penetrate into its subterranean 

 abode." The author then adds the following observations, with which our own observations on 

 the eyes of Gambarus pellucidus and hamulatus do not at all agree: " The cornea also exhibits an 

 appearance of being divided into a few. imperfect corneales at the apex of the organ (Fig. 14), and 

 the structure behind these into chambers, to which a small but distinct optic nerve is given 

 (Fig. 13 d d)." 



Mr. Newport's specimen must have been quite imperfect, as we have been unable to find any 

 " corneales ; " nor such a termination of the optic nerve as the distinguished author figures. Nor 

 were such noticed by Leydig, whose work we now proceed to notice, giving an abstract of Chapter 

 III, " Eyes and Antennae of the Blind Crayfish of Mammoth Cave." 



" Gambarus pellucidtis Tellkf., known as * the blind crayfish of Mammoth Cave,' must likewise 



