97 G SUMMARY OF CURRENT RESEARCHES RELATING TO 



1. The lens is circular in the principal eyes, often elliptical in tho 

 secondary. It is biconvex, and the inner hemisphere more than the 

 outer. The cornea is continuous with the general cuticle, and like it 

 exhibits concentric lamellae of different thickness and refractive index. 

 The outer layer, which is continuous with the pigmented stratum of 

 the cuticle, is generally colourless, but even more pigmented in 

 Attidae. As in the cuticle, fine canals traverse the lens radially at 

 right angles to the surface, disappearing, however, in the centre. In 

 some lenses a spherical body of undetermined import projects inter- 

 nally, with thicker and more equal layers. The whole lens is pene- 

 trated by the perilymph fluid. 



2. Vitreous body and pigment-cells. — As the corneal lens is continu- 

 ous with the cuticle, so the pigment-cells and vitreous body represent 

 a modification of the hypodermis, which is in other regions very 

 variable. The pigment-cells round about the lens are long and 

 narrow, with slight cell-walls, finely granular striated protoplasm 

 packed with pigment, and a small fusiform central nucleus. The 

 vitreous body usually consists of polygonal, truncated, pyramidal 

 cells, with very strong cell-walls and yellowish refractive content. Tho 

 cells are variously disposed more or less symmetrically beneath the lens. 



3. The retina (a) of the principal eyes. The nerve is rarely in 

 the axis of tho eye, but usually lateral. It divides within the bulb 

 into several branches, of which one, retaining the old direction, 

 traverses the retina, running often just below the preretinal lamella, 

 and uniting with the retinal cells at the opposite end, while the other 

 branches pursue various courses. The fibres, which, even towards 

 the bulb, have a tubular disposition, probably form a funnel-shaped 

 expansion towards the retinal cell, and while the wall of the former 

 unites with that of the latter, the protoplasmic networks of the two 

 probably also fuse. The nerve-cells are long and flask-like, disposed 

 perpendicularly to the wall of the optic bulb, so that those in front 

 are almost parallel to the preretinal lamella. The large spherical 

 nucleus lies near the base of the cell where the nerve-fibres enter. 

 At the narrowed end, next the preretinal lamella, the "rods" are 

 seen. They are simply the modified peripheral ends of the cells, 

 though sometimes more or less distinct. Other features, such as the 

 chiasma (in Lycosidae at any rate), are then briefly discussed. 



b. The retina of the secondary eyes. — Prof. Bertkau devotes most 

 attention to the tapetum, which is not represented in the principal 

 eyes of the true spiders. It forms in the Sparassidae a connected 

 layer traversing the posterior portion of the optic bulb. Two flaps, 

 united at their ends, inclose a long funnel-shaped space, which may 

 be crossed by narrow bridges. In the Lycosidae and Thomisidae 

 numerous parallel clefts have been formed at regular intervals, 

 extending from 1/5-1/3 of the breadth of the tapetum. The system 

 of bands which thus results has a sort of grate-like appearance. The 

 fine structure varies greatly, but no complete account is yet forth- 

 coming. The crystalline particles which give it its beautiful sheen 

 are discussed at some length. Bertkau regards the luminous layer 

 as a kind of secretion. 



