10 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Figs. 30 and 31 represent enlarged views of the rudimentary eye of two difierent specimens of 

 G. Htygia from Mammoth Cave. In the sections represented by Fig 30 a h we see that the number 

 of fiicets has been reduced apparently to two (6), the rudimentary lenses being enveloped by a 

 black pigment mass. This section, examined by Tolles' i A, is magnified and drawn to exactly the 

 same scale as that of the eye of Asellus represented by Fig. 21. In that figure may be seen the 

 normal size of the lenses and of the retina cells. It will be seen that in Ca^cidotiiea 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. 31a and 32a, it will be seen 

 that the eyes in one are considerably larger than in the other specimen. Fig. 32& 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 32cl on the edge of the eye one lens is indicated (though the divisions 

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

 at Fig. 32e. The four sections Or-d passed through the eye, the section in front and behind not 

 touching the eye itself. 



It thus.appears from the observations here presented that the syncerebrum of the blind Cseci- 

 dotiea 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 trans- 

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

 but usually microscopic in size, and about one-fifth as large as that of the noi'mal 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 fonn, the loss of the lens and pigment. 



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

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

 tacea, as well as insects, 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 oi'igination of these forms, rather 

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



BIBLIOGRAPHY OF WORKS ON THE NERVOUS SYSTEM OF CRUSTACEA. 



GENERAL. 



Bellonci, G. Intorno alia struttura e alle connessioni dei lobi olfattoril negli Artropodi snperiori e nei verte- 

 brati. 2 tav. 4. Ivi, 1882. 



Berger. Untersuchungen uber den Ban dea Gehirns und der Retina der Arthropodeu. Arbeitea des zool. Insti- 

 tuts zu Wien, Heft 2. 



Nachtrag zu den Untersuchungen iiber den Ban des Gehirns und der Retina der Arthropoden. Ibid., 



Heft 3. 



Ehrenberg. Beobachtnng einer bisher unerkannten Structur des Seeleuorgans. 1836. 



Grenacher, H. Untersuchungen Uber das Seeorgan der Arthropoden, insbesondere der Spinnen, lusecten, und 

 Crustaceen. Gottingen, 1879. 



Hannover. Recherches microscopiqnes sur le systfeme uerveux. 1857. 

 Helmholtz. De fabrica systematis nervosi evertebratorum. Diss. Berolini, 1842. 

 Leydiq. Lehrbuch der Histologic des Menschen und der Thiere. 1857. 



Vom Bau des thierischen Korpers. Erster Band. 1804. 



Tafeln zur vergleichenden Anatomie. Erstes Heft. 1864. 



Milne-Edwards, H. Histoire natnrelle des Crustac^s. Tom. i-iii. Paris, 1834-1840. 

 Re.mak. Uober d. Inhalt der Nervenprimitivriihren. Archiv f. Anat. u. Phys., 1843. 

 Walter. Mikroscopische Studien iiber das Centralnervensystem wirbelloser Thiere. 1863. 



