REPORT ON THE MOLLUSCA. 33 



Among Crustaceans, modifitations occur in the two directions mentioned above in 

 respect to pelagic animals in general, that is, towards atrophy and towards hypertrophy. 

 An example of the latter is furnished by Cystisoma neptuni, Gu^rin ; and such modifi- 

 cations are probably useful for the perception of luminous rays which would be imper- 

 ceptible to normal eyes, either on account of their feeble intensity, or because of their 

 special chemical character. Examples of such modification are not, however, numerous, 

 and there is no instance of a Gastropod from the great depths in which the visual 

 organs have undergone such a change. 



In all Gastropods from that habitat the eyes have been markedly atrophied (as 

 in Guivillea), or liave totally disappeared, as in the other species discussed after 

 Guivillea (Pleurotoma lepta, Phurotoma hrycliia, Fossarus{}.) cereus, Puncturella 

 hrychia). Some other instances of the latter condition are already known in species 

 from various depths, and I am convinced that further researches will greatly increase 

 the list. 



A. Pleurotoma nivale, Loven,' of which G. 0. Sars has made a special genus, 

 under the title TyphlomangUia,' lives at about 170 fathoms. 



B. Fusus abyssorum, Fischer,' collected from between 1300 and 2800 fathoms. 



C. Eulima stenostoma, Jeffreys,'' lives at about 90 fathoms. 



D. Tectura fidra, 0. F. Mliller, sp. 



E. Lepeta s. str. 



F. Propilidinm. 



The markedly rudimentary character of the eyes of the subterranean or abyssal 

 Molluscs is produced in an entirely different fashion from that which we have noticed 

 in certain Gastropods (burrowers, some Nudibranchs, &c.), where the organs are 

 concealed under the skin, and undergo diminution in size. In the present instances 

 the eyes remain on the surface and do not become reduced in size, but lose successively 

 certain of their constitutent portions, or altogether disappear. 



Among the Gastropods from great depths, as among the subterranean forms, there 

 are several {Troclms infundihidum, Trochus rJiina, Tiirho transenna, &c.) which still 

 retain well-pigmented eyes, like those of littoral species. Like some of the cave 

 forms above referred to, they are instances of more recent and still incomplete adaptation. 

 It is certain none the less that abyssal Gastropods, as well as the subterranean forms, 

 have a general tendency to become rudimentary and to lose their eyes. 



' Jeffreys, British Conchology, vol. iv. p. 389. 

 -' MoUusca regionis arcticse Norvegiae, p. 241. 



' F'ischer, Sur les especps de Mollusques arctiques trouvees dans les grandes profondeurs de rOc&n Atlantique 

 intertropical, Comptes reiuln.i, xcvii. p. 1498. 



' Jeffreys, British Conchology, vol. iv. pp. 207, 208. 

 (zooL. CHALL. EXP.— PART Lxxiv.— 1888.) Eeee 5 



