446 F. R. Cowper Reed — Blind Trilohites. 



primitive or characteristic larval structures, and we cannot for 

 the same reasons consider them as rudiments of a structure 

 once generally present in the ancestors of the trilobites. Their 

 exact nature in the case of Harpes is still not fully determined. 

 Packard ^ does not regard them as comparable with the simple eyes 

 or ocelli of Limulus, and Clarke - is inclined to think that they may 

 prove to be of an aggregate character similar to the schizochroal 

 eyes of Phacops. If the latter is the case we have an interesting 

 example of parallel independent evolution. On the whole, it appears 

 safer to consider the eye-spots in Harpes as the attempt on the 

 part of a primitive type of trilobite to develop organs of vision 

 which might enable it to hold its own in the struggle for existence 

 with its more highly organized contemporaries, which possessed 

 well-developed compound eyes. Similar conditions of life required 

 similar organs of sense, and the primitive types which failed to 

 develop them ceased to exist. Only those of which the habits 

 removed them from the conflict, or which were able quickly to 

 develop the necessary powers of defence or flight, managed to 

 survive. In support of this view the long range of the genus 

 Harpes may be adduced, for it appears in the Cambrian and lasts 

 till the Devonian. 



In the case of the Trinucleidge, none of the genera in their 

 adult condition possess organs of vision; all are blind. But some 

 immature Trinuclei have eye-spots, as already mentioned. McCoy ^ 

 was led by the occurrence of these eye-spots in certain individuals 

 to institute a new genus which he called Tretaspis. It has, however, 

 been abandoned,* since it is believed to be proved that it was 

 founded on larval forms of species which in the adult condition 

 have lost the 'ocular' tubercles. The remarks made on the eye- 

 spots of Harpes apply partly to these in the larval Trinucleus. 

 Thus we cannot regard them as an inherited character, nor as 

 a primitive or larval structure common to lowly types of trilobites. 

 It only remains to consider them as special generic or perhaps specific 

 features, peculiar to the ontogeny of Trinucleus or of some of its 

 species. They appear, therefore, to be adaptive characters, acquired 

 to meet certain conditions of life; and in explanation of their absence 

 in the adult we may either suppose that the adult had a different 

 environment and mode of life which led to their degeneration, or 

 that other organs developed which played an equivalent part, as 

 is the case of the tactile organs of blind cave-animals. It may 

 be mentioned that the visual function of these tubercles in the 

 larval Trinucleus has never been demonstrated, and no description 

 of lenses or visual surface has ever been published, so far as 



1 Amer. Nat. (1880), p. 503. 



* Journ. Morph., vol. ii (1888), p. 253. 



3 Ann. Mag. Nat. Hist., vol. iv (1849), p. 410; and Brit. Pal. Foss., 1851, 

 p. 146. 



* Barrande, Syst. Sil. Boh., vol. i, pp. 617, 620. Nicholson and Etheridge, 

 Mon. Silur. Foss. Girvan, fasc. 2 (1879), pp. 188-197. Beecher, Amer. Journ. Sci., 

 vol. xlix (1895), p. 307. 



