MUSEUM OF COMPARATIVE ZOOLOGY. 93 



son assigned, untenable. It is more likely that the primitive baciUi 

 have, with loss of function, atrophied, and that consequently the pre- 

 nuclear bacilli of inverted retinse are not homologous with the pre-nuclear 

 bacilli of unin verted retinse. 



It is possible that the primitive bacilli do not in all cases completely 

 atrophy. There are at least certain problematic bodies in the retina of 

 scorpions which may find an explanation in connection with this hy- 

 pothesis. I have in mind the structures which Graber described for 

 Audroctonus as " posterior nuclei," — subsequently claimed by Grenacher 

 ('80, pp. 423, 424) to be only peculiar, highly refractive bodies, — and 

 the structures which Lankester and Bourne ('83, pp. 185, 193) have 

 seen in the central eyes of Euscorpius Italicus, and have described under 

 the name of " phaospheres." 



It may be an obstacle that the " phaospheres " are also sometimes 

 found in front of the nuclei, and further, that the rhabdomeres are not 

 formed within, but at the surface of the retinal cells. The variability in 

 the relation of the phaospheres to the nuclei may be regarded as an ab- 

 erration rendered possible by the loss of function, rudimentary structures 

 being more liable to vary than such as are at the height of their func- 

 tional activity. (Compare Darwin, Origin of Species, chap, v.) The 

 second obstacle is probably not of great importance, since it still remains 

 to be shown that intra-cellular and extra-cellular rod-like structures are 

 essentially different. Besides, it is conceivable tlaat the primary bacilli 

 may have been intra-cellular, while the secondary bacilli are extra- 

 cellular. 



A more serious obstacle arises from the fact that similar structures 

 (phaospheres) also exist in the lateral eyes of Euscorpius Italicus (Lankes- 

 ter and Bourne, '83, p. 185), in the case of which, evidences of an in- 

 folding and inversion are not so satisfactory as with the median eyes. If 

 the lateral eyes do not result from an infolding and inversion of the re- 

 tinal layer, this explanation of the " phaospheres " would go for little or 

 nothing, since their presence in the lateral eyes could not be explained 

 on the same hypothesis. I have endeavored, however, to show (p. 59) 

 the great probability of an inversion of the retina in these lateral eyes, 

 and must await a satisfactory disproval of that opinion before allowing 

 this possibility to outweigh the considerations in favor of the explanation 

 of phaospheres which is here attempted. 



In the above hypothesis regarding the origin of "pre-nuclear" ocelli, 

 tlie two points demanding explanation have been kept in view, — the 

 continuance of functional activity, maintained by means of the simul- 



