MUSEUM OF COMPAKATIVE ZOOLOGY. 61 



have hitherto received. He has especially defended the cuticular inter- 

 pretation of the " sclera," and in connection therewith has urged the ex- 

 istence of a cuticular matrix. The nuclei of this matrix he has very 

 distinctly, and I am inclined to think very truthfully, figured (Fig. 18, k) 

 and described (pp. 64, 84) for Scolopendra. Even Grenacher ('80, p. 441) 

 has granted a conditional assent to their presence, although maintaining 

 that he did not feel entirely convinced.* 



To anticipate a conclusion, the grounds of which will be presented 

 later, — in connection with a discussion of the nature of the pre-retinal 

 membrane, — I may say here that the existence of a distinct cell-layer 

 posterior to the retina, and inside the cuticular " sclera," appears to me a 

 strong argument in ftivor of the view that the retina in the Scolopendridse 

 has been formed by an involution with inversion. If Graber had realized 

 the probable identity of these posterior cell-layers in Myriapods and 

 scorpions, it is possible he might have been saved the expression of his 

 sixth conclusion : " The ends of the retinal sacs [cells] appear to form, 

 at least in part, the matrix of the sclera." 



There is a very palpable difference between the figures of the " vitre- 

 ous" by Graber, and the figures and descriptions by Grenacher ('80, 

 p. 434) ; nor is there any room to doubt that Grenadier's work is, in 

 most particulars, incomparably the more satisfactory and reliable. But 

 Grenacher finds, if not a layer of uniformly fashioned cells, at least in 

 some individuals of one species (Branchiostoma) a vitreous composed 

 of an uninterrupted layer of cells, which differ from the vitreous cells 

 of spiders, for example, only in the more central position of their nuclei, 

 and the inclination of their axes towards (deep ends away from) the axis 

 of the eye. This exceptional condition of the vitreous — found only in 

 a few individuals — Grenacher brings into relation with the fact that the 

 lens in these cases Avas only partially developed, and deduces the con- 

 clusion that these animals had recently suffered a moulting, and that the 

 increased thickness of the hypodermis and vitreous is simply evidence of 

 increased functional activity. He recognizes the difficulty in the way of 



* According to Grenacher ('80, Fig. 8) the pigment-cells which invest the eye 

 have the character of a continuous epithelium such as the posterior layer of the re- 

 tinal infolding in spiders does at an early stage ; but their relation to the thick strati- 

 fied cuticula (viz. outside the latter) forbids a comparison. If Grenacher's account is 

 correct, the ilyriapods stand quite alone in having such a continuous mesodernuc 

 investment of the eyes. 



Compare also Sograff ('80), PI. Ill, fig. 17, where a nearly continuous layer of 

 cells is represented outside the thick cuticula of the eye, but inside only isolated 

 nuclei scattered among the nerve-fibres which occupy the space between the cuti- 

 cula and the basal ends of the retinal cells. 



