THE EYE IN CERTAIN CYMOTHOIDiE. 449 



bounding the ommateum ; in sections I have recognised these fibrils in the 

 interior of the hyaline cells, but have not been able to trace them beyond the 

 membrane into the tissue of the ganglion ; indeed, in sections. I never suc- 

 ceeded in finding the fibril in the space between the hyaline cells and the 

 membrane bounding the ommateum ; the extension of the fibre, as far at least 

 as this membrane, was perfectly obvious in teased preparations, both depig- 

 mented and undepigmented. In the Cymothoidse I did not observe the fibrils 

 in the interior of the hyaline cells, but in transverse sections of the prolongations 

 of the retinula cells beyond the membrane of the ommateum there was very 

 frequently an axial body (fig. 13 #.), which may very possibly be this same fibre. 

 The nature of this axial fibre would be hard to determine merely from a 

 study of the eye of the adult crustacean. Fortunately, however, I have been 

 able to examine the eyes of some very young examples of Serolis schythei taken 

 from the brood pouch of the mother ; I had not completely studied these 

 specimens before preparing my memoir on Serolis. In some of these the hyaline 

 cells appeared to be fully developed, and the ommatidia only differed from 

 those of the adult in their smaller size. The vitreous body as completely filled 

 the interior of the vitreous cells as in the adult, showing that the increase in 

 size of the vitreous body advances pari passu with the growth in size of the 

 vitreous cells. In other specimens, indistinguishable in point of size, I was 

 unable to detect the hyaline cells. In the depigmented teased preparations 

 (see fig. 19) the retinula cells appeared to be close together, and in the axial 

 space was a delicate fibre, appearing to branch superiorly into a number of 

 fibrils ; this bundle of fibrils lies between the upper ends of the retinula cells 

 and has a conical form ; the interspaces are filled with granular matter. The 

 whole structure appears to be the rhabdom ; in my figure of an ommatidium 

 of Serolis (loc. cit., pi. ix. fig. 5), I have evidently sketched a rhabdom of a 

 young specimen, and erroneously associated it with the ommatidium of an 

 adult. Outside the rhabdom, in the immature eye, are thickenings of the 

 margin of the retinula cells (figs. 17, 18) ; these have a very different appear- 

 ance from the enclosed rhabdom. They ultimately become the structure illus- 

 trated in figs. 2, 3r, which I have regarded as the rhabdom.* 



* Grenacher has not noted the presence of special pigmented cells within the ommateum of 

 Isopods. In Serolis there are two or three rows of such cells surrounding each element of the eye ; the 

 cells themselves are not easily to he made out, hut their nuclei are particularly distinct in teased prepara- 

 tions, when the pigment has been dissolved away by nitric acid. I am not certain as to the exact 

 number of these nuclei in each element ; the action of the nitric acid is such that the cells, of which 

 these are the nuclei, are not merely depigmented, but are entirely dissolved away. I could never dis- 

 cover any traces of the cell protoplasm in such preparations ; although these nuclei often appear on a 

 superficial inspection to lie in the retinula cells, carefulo cu ssing shows that this is not the case ; more- 

 over, the nuclei themselves are smaller than, and in other respects different from, the nuclei of the 

 retinal cells. 



