Eyes of Molluscs and Arthropods. 621 



it witli the vitreous body. His conception of an eye must be something 

 altogether iinique. for, after having described. in the eyes of Patella, 

 the vods. which he erroueously considers as a vitreous body, aud the re- 

 tineum. — having the same structure as in other Mollusca, — he in- 

 forms US that the orgau in question is no eye at all, but simply a pig- 

 meuted hollow in the epidcrmis! Then, immediately afterwards, he 

 speaks of the pigmented cells as »Siuneszellen« or ))Sehzellen«, and 

 considers the thickened cuticula, secreted by the colorless cells, as 

 being speeially adapted to protect the very delicate ends of the rods. 

 We should be pleased to know what a pigmented hollow, provided with 

 a true retina, and containing delicate seeing cells, is, if it is not an eye. 

 He evidently considers that these organs have too large an openiug to 

 be entitled to be called eyes, for he does not hesitate to apply that 

 name to an exactly similar structure ìnHaliotis, the opening of which is 

 of more modest dimensions I In the latter genus, he has mistaken the 

 retinidial layer for the vitreous body, which, in his preparations, seems 

 to have entirely disappeared, with the exception of the lens-like 

 thickening covering the opening of the optic cup. The colorless Cle- 

 ments are not secrete-cells , neither do the pigment cells contain any 

 colorless axis which, when continued beyond the limits of the pigment, 

 forms the rods. The fibres, supposed to be secreted by the colorless 

 cells , instead of being simply formless cuticular secretions to be 

 transformed into the vitreous body, are rods, similar to those of 

 the pigment cells, and supplied in the same manner with nerve 

 fibres. 



HiLGER. who has recently made a valuable contribution to our 

 knowledge of the retina in various forms of Mollusca, has found that 

 the colorless cells are generally, aud probably universally present in the 

 retineal layer. He has abandoned the old method of considering them 

 as scerete, or support cells, and, recognizing the part they play in the 

 construction of the rods, calls them the rod cells, although it is difficult 

 to understand why, for, according to his figures, they contribute much 

 less thau the pigment cells to the composition of the rods. He has con- 

 sidered the retina oiHaliotis in less detail than that of other forms with 

 which I am unacquainted. I believe , however , that he is wroug in 

 supposing that the rods of the severa! pigment cells , surroundiug the 

 Prolongation of a central colorless one, unite to form a single rod. It 

 seems to me much more probable that the same condition, — similar to 

 that found in Haliotis, as regards the structure of the retinidial 

 layer, — prevails throughout all the Mollusca with equally simple 



