lOB MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 



Figs. 30 and 31 re.preseut eularged views of the riidiiueatary eye of two diflerent specimeus of 

 C. stijijiii from Maiiiinoth Cave. In the sections represented by Fig 30 a h we see tliat the number 

 of f;icets lias been reduced apparently to two (b), the rudiiuentary lenses being enveloped by a 

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

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

 uormal size of the lenses and of the retina cells. It will be seen that in C;eci(lot;ea the retina cells 

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

 l)ortion we sujjpose 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, ola and 32a, it will be seen 

 that the eyes in one are considerably larger than iu the other specimen. Fig. 32/; shows that in 

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

 At the i)oint indicated by 32(Z on the t dge 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 

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

 touching the eye itself. 



It thus appears from the obsei'vations here presented that the syncerebrum of the blind Ca'ci- 

 dota'a 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 i)igmeut mass inclosing very rudimentary minute lens-cells, which have lost their trans- 

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

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



TLe steps taken in tlie 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 Ctecidotiea 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 origination of these forms, rather 

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



BIBLIOGRAPHY OF WORKS ON THE NERVOUS SYSTEM OF CRUSTACEA. 



GENERAL. 



Belloxci, G. Intoruo alia struttiua e alle coiinessionl del lobi olfattorii negll Artropodl suiieiiori e iiei veite- 

 lirati. 2 tav. 4. Ivi, 1882. 



Berger. UntersucliUDgeii Uber den Bau des Gehirus uud der Retiua der Artbropodeu. Arbeitou des zool. lusti- 

 tiits zu Wieii, Heft 2. 



Nachtrag zu deu Uutersuchuugeu uber deu Bau des Gehirus uud der Retiua der Artbropodeu. Ibid., 



Heft :?. 



Ehrenbeug. Beobachtung ciuer bisber unerkauuteu Strnctur des Seeleuorgans. 1836. 



GUENACHEH, H. Untersucbuugen iiber das Seeorgau der Artbropodeu, iusbesondere der Spinueu, lusecteu, iiud 

 Cru.staceen. Gilttingeu, 1879. 



Hannover. Rechercbes microscopiques sur lo systeme uerveux. 1857. 

 Hr;LMHOLTZ. De fabrica systeniatis nervosi evertebratoruni. Diss. Berolini, 1842. 

 Leydig. Lebrbucb der Ilistologie des Mensclien uud der Tbiere. 1857. 



Vom Bau des tbieriscbeu Korpers. Erster Baud. 1864. 



Tafelu zur vergleicbendeu Auatoiuie. Erstes Heft. 1864. 



Milne-Edwards, F1. Histoire uaturelle des Crustacfe. Tom. i-iii. Paris, 1834-1840. 

 Remak. Ueber d. lubalt der Nervenpriuiitivruhren. Arcbiv f. Anat. u. Pbys., 1843. 

 Waltek. Mikroscopiscbe Studieu iiber das Centralnervensystem wirbelloser Tbiere. 1863. 



