558 F. R. Coiqoer Reed — Blind TriloUtes. 



seems that their mode of life here leads to just the same modifications 

 as the deep-water conditions. It is not, indeed, improbable that the 

 similar mode of life in many deep-sea forms has more to do directly 

 with their blindness than the fact that light is practically or nearly 

 absent in the abysses of the ocean. Thus Agassiz (loc. cit.) says that 

 the deep-sea blind fishes belong to families with burrowing habits, 

 and the same may be said of the blind Crustacea and Gastropoda. 

 Semper^ has pointed out that many blind animals live in well- 

 illuminated situations. 



Many Isopods are blind in shallow water as well as in the deep 

 sea (e.g. Munnopsis and Eurycope). Pleurogonium is quite blind in 

 20 fathoms of water, and the much quoted blind species Pelalo- 

 phthalmus armtger ranges from 140 to 2.285 fathoms, and Psendomma 

 JSarsi from 110 to 1,500 fathoms.- The forms which grub in the 

 mud have frequently no eyes,^ while free-swimming allied species 

 have well-developed eyes. Marshall* remarks that it is very strange 

 that the blind Schizopod Psendomma australe lives in only 33 fathoms 

 of water, but he does not give any account of its habits. 



Indeed, benthonic conditions of life seem to lead to reduction or 

 loss of eyes, whether the organisms live in deep water or shallow. 

 Packard (loc. cit.) suggests that it may be found that the deep-sea 

 forms without eyes burrow in the ooze or live under loose objects 

 at the bottom, thus being subjected to conditions which may 

 be completely paralleled in shallow water, and which the above 

 examples seem to show produce the same results. 



Semper (loc. cit.) also mentions a fact showing that other causes 

 as well as the absence of light lead to the loss of the eyes. Thus, 

 in the cave-beetle Machcerites the females are blind, but the males 

 have well-developed eyes. It is also a well-known fact that the 

 free-swimming larvae of parasitic Crustacea and the Holothurians 

 possess organs of vision, whereas the adults have no eyes, though 

 frequently living in well-illuminated portions of the sea. The 

 mode of life has made eyes superfluous, and therefore they have 

 been lost. 



Turning now to the blind trilobites to see how the above facts 

 derived from living Crustacea apply to them, we notice that the 

 members of our Group 2 — i.e. the ' adaptive forms ' — have not 

 a wide distribution ; they are mostly local forms found only in 

 a small and restricted area. The wide distribution of modern deep- 

 sea species finds therefore no analogy in them ; and the statement 

 that it does is due to the inclusion of those phylogenetically blind 

 forms which belong to our Group 1. Incidentally it may be 

 remarked that the wide distribution of these phylogenetically blind 

 ^enera may be due to the same cause as that which causes the wide 

 ^j^' stribution of modern deep-sea organisms, and this cause is the 

 T:>(,''^^ormity of physical conditions, particularly of temperature, which 



oorr.^,^ 1 ''Animal Life" (Internat. Sci. Ser.), pp. 76-87 and 419-421. 



blind -2 Walther, Einleitung in die GeoL, etc. (Jena, 1893-4), pp. 43, 44. 



ar"^ 1 ■^' Sars, Chall. Eep., vol. xxxvii. 



^^- "Die Tiefsee und ihr Tierleben, 1888, p. 26-5. 



