Eyes of Molluscs and Arthropods. 683 



of the sensitive layer, and the general structure of the eye, was. It 

 is supposed that the primitive Arthropod eye was formed of three layers 

 (PI. 32, tig. 141), and that all other forms are modifieations of this type. 

 Even if we admit that this supposition is true, our knowledge of these 

 layers is too small to be used at present with any eflfect. Although 

 not of primary importance, the evidence obtained from a study of 

 these threc layers may be used in conjuuction with that obtained 

 from a knowledge of the ommatidia. which must be regarded as 

 the elements possessing the greatest elassifying value. Upon them, 

 I believe, must be founded the Classification of all eyes, and not 

 upon the number of layers, or upon the lens, or any similar part 

 of equally small, physiological, or morphological value. Lankester's 

 term retin u late, signifying «an ommateum in which the nerve end 

 cells are segregated to form definite groups, or retinulae«, is not ad- 

 missible , since it is founded upon a wrong conception of the struc- 

 tural elements of the eye. All eyes are retinulate in this sense, 

 since they consist of ommatidia formed of compound retinophorae sur- 

 rounded by pigment, or ganglionic cells. 



The term »vitreousbodywisa bad one, since he uses it to desig- 

 nate a layer of cells, whereas the termhas already been universally 

 used to designate a non-cellular, vitreous secretion. Moreover he 

 applies it to »the anterior cell wall of a diploblastic ommateum« ; it is 

 certain that in the compound eye this layer (the corneal hypodermis) 

 does not scerete any vitreous body, but gives rise to the corneal facets 

 alone. It is also extremely probable that the homologous layer in the 

 stemma and ocelli, or the vitreous body (which ought to be called the 

 corneal bypodermis), likewise gives rise to the corneal leus, but to no vit- 

 reous secretion; this function is performedby the outer wall of the primitive 

 optic vesicle, or by the median of the three ectodermic layers. To this 

 layer, which is entirely absent in the compound eye, I bave given 

 the name vitreous layer, and to that immediately underlying the 

 corneal cuticula, that of corneal hypodermis; the latter cannot fall 

 to be present in ali Arthropod eyes, possibly with the exception of 

 certain Crustacea, such as Copepods, Cladocera etc. which we bave 

 not included in any of our previous Statements. 



We cannot recognize the term »vitrella« , or »a group of cells of 

 a vitreous body which has become segregated in correspondence with 

 the segregation of the retinal body and of the lens«, siuce it is founded 

 upon what we consider to be a false conception of the structure of the 

 compound eye. 



