440 INVERTEBRATE MORPHOLOQT. 



the outer ends are situated a number of rodlike bodies (rhab- 

 doms, &), whence these eyes have been termed prebacillar; 

 the nerve-fibres are continuous with the inner ends of the cells. 

 In the posterior dorsal and lateral eyes {B) an inversion of 

 the retina (r) has taken place, so that the rods (6) are situated 

 at the apparently inner ends of the cells and the nuclei at 

 their outer ends, whence the term postbacillar applied to 

 these eyes. The optic nerve-fibres enter at the sides of the 

 eye and are distributed to the nuclear ends of the retinal 

 cells, recalling the arrangement occurring in Pecten among the 

 Mollusca. The innermost layer of the eye upon which the 

 ends of the rods rest is cellular, numerous minute crystals 

 being deposited in the cells, whence it has the function of a 

 reflector and is termed the tapetum (t). It is quite wanting in 

 the prebacillar eyes. 



The significance of the structure of the Arachnid eye may be under- 

 stood by supposing it to have been derived from a compound eye similar to 

 that of Limulus (see p. 431), the individualities of the various ommatidia 

 being more or less subordinated. The cuticular cornea in Limulus is 

 smooth upon its outer surface, the inner surface being produced into 

 papillse, one of which corresponds to each ommatidium. In the Arachnids 

 even these papillae are wanting, the cornea showing no evidence of the 

 presence of ommatidia. The lateral eyes of the Scorpions approach more 

 nearly in their general structure the eyes of Limulus, though the conden- 

 sation of the ommatidia has been carried further than in the median eyes 

 of that form, or in the posterior dorsal and lateral eyes of the Spiders. But 

 in these eyes the condensation is associated with an invagination of the en- 

 tire eye, a process which, it may be remarked, is indicated in the median 

 eyes of Limulus. This invagination has been regarded as a pushing in, 

 under and parallel to the hypodermis, of a pouch of that layer, a process 

 which gives in cross-section the appearance of an S-shaped fold. The 

 outermost layer of the fold forms the vitreous cells or corneal hypodermis, 

 the middle layer the retina, the inversion of which is plainly seen in the 

 posterior dorsal and lateral eyes of the Spiders, while the innermost layer 

 forms the postretinal layer in the Scorpions and the tapetum of the 

 Spiders. The ommatidial retinulae are more or less retained in these eyes, 

 as is shown by the structure of the rhabdom, which in the Scorpions is 

 composed of five ' parts, in the Spiders of two, and in the Harvest -spiders 

 of three. The anterior dorsal eyes of the Spiders do not seem to have 

 undergone an invagination, hence the absence of a tapetum and the preba- 

 cillar structure of the retina; a corneal hypodermis is, however, present 

 and would seem to indicate an invagination, but its mode of origin seems 

 at present but imperfectly understood. If a generalization is to be made it 



