32 THE VOYAGE OF H.KS. CHALLENGER. 



3. In many animals belonging to various groups living in the absence of liglit, it 

 has been observed that the eye is rudimentary and without vision, or that it has entirely 

 disappeared. 



Among the anisopleural Gastropods which are in this condition we may cite, — 



A. The species of Caecilianella (for example, CiecilianeUa acinda, 0. F. MuUer, sp.), 

 which live concealed in the ground. In these the absence of pigmented eyes has been 

 known since Nilsson's investigation.' In the species of Testacella, the eye, although 

 very minute and almost rudimentary, is still distinct and pigmented ; and this not 

 unnaturally, since this Mollusc has not an exclusively subterranean existence. 



On the other hand, it is well known that a certain number of animals, including both 

 terrestrial and fresh-water forms (Amphibians, Fishes, Insects, Arachnids, Crustaceans, 

 Molluscs, &c.), live in caves absolutely shut off from the light. They form the "cave 

 feuna" of Carniola, Falkenstein, of the mammoth cave of Kentucky, of Cuba, &c. 

 Among the anisopleural Gastropods which are found blind in such environment, we 

 may mention, — 



B. The species of Zosj^eum, allied to Pupa? 



C. Helix hauffeni, F. Schmidt.' 



D. Bithinella pellucida, Hauffen, sp. 



The last species, investigated by Wiedersheim* and de Rougemont," exhibits at the base 

 of the tentacles an unpigmented tubercle, like that of GuivillecC 



Among the Gastropods of the cave fauna there appear to be some forms which 

 still retain normal or approximately normal eyes. Such, for example, is a species 

 of Zonites described by Dall.'^ These are forms in which adaptation to a life in dark 

 caves has not yet l)een so old-established or so complete as that of the species above 

 mentioned. 



4. Finally, there are certain marine animals (Fishes, Crustaceans, IMolluscs, &c.) 

 which live in depths so great that the light is feeble or nil, apart from that produced 

 bv phosphorescent forms. These also exhibit marked modifications in the organs of 

 vision. 



' Historia Molluscorum Suec'ae, 1823. 



- Frauenfeld, Besuch einigcr Krainenbohlen, Verh. d. k. k. zool.-hot. Verehis Wicn, Bd. iv. p. G4, 1854 ; Frauen- 

 feld, Die Giittung Carychium, IhiiL, p. 75. 



" Schmidt, Beschreibung ueuer Hohleutliiere, f'erli. d. k. k. zoul-hot. Verews TT'iVn, Bd. v., 1856, p. 4. 



^ Beitrage zur Keuutuiss der Wiirttembergischen Hohlenfauna, Vcrh. d. Phys. Mtdic. Gts. Wlir:hur(j, Keue Folge, 

 Bd. iv., 1873. 



^ fitude sur la faune des caiix privces de lumiere (1876). 



'■ Wiederslieim, loc. cit., p. 210, pi. vii. fig. 14, h. 



' Packard, Ou a uew cave fauna in Utah, Bull. U. S. Gcol. and Geofjr. Survey of the Territories, vol. iii., 1877, ji. 

 103. 



