416 MR. B. THOMPSON LOWNE ON THE COMPOUND VISION 



the vitreous is said to be absent in the lateral eyes of Scorpions, although it is present 

 in the more highly developed central eye. This is a remarkable exception to the 

 conditions which I have observed ; but as I formerly overlooked the vitreous in cases 

 in which I now know it exists, I cannot help suspecting that the cells have been destroyed 

 in soine way in the preparation of the sections. 



I propose the following classification of the visual organs of the Arthropoda. 



I. Simple ocelli. III. Aggregate eyes. 



II. Compound ocelli. I.V. Compound eyes. 



I. Simple ocelli. — I include under this term the ocelli of Arachnida aud the 

 stemmata of perfect insects, and I think it probable that the eyes of the Myriapoda 

 consist of clusters of such stemmata ; but at present I am not sufficiently acquainted with 

 the modifications of the eyes of this family to speak with any degree of certainty, as the 

 investigation of the eyes of Myriapods is exceedingly difficult. 



II. Compound ocelli. — I use this term to indicate the ocelli of larval insects in 

 which there is apparently a second refractive system — the spindle, which magnifies and 

 erects the subcorneal image. 



I have little doubt that the eyes of the Coryceidce, at least those of Cojjilla, belong to 

 this class ; but I have not had any opportunity of examining them, and only judge from 

 Dr. Grenadier's description and figures. 



III. Aggregate eyes. — I include in this division the semi-compound eyes of Isopods, 

 which appear to be nothing more than aggregations of compound ocelli. 



Dr. Grenadier regards the subcorneal lens of the Isopod as a highly modified crys- 

 talline cone. I regard it as the representative of the oil-lens of the compound eye, and 

 of the lens of the compound ocellus ; perhaps the cone of the compound eye should also be 

 regarded as a highly modified form of lens ; at least it departs more from the primitive 

 type than the spheroid lens of the Isopod. 



IV. Compound eyes. — These are the ordinary eyes of the Crustacea and Insecta. 

 They exhibit very various forms, and many efforts have been made to classify them, 

 chiefly founded on the variation of the dioptron, and especially those of the cornea and 

 crystalline cone. 



All these classifications appear to me unsatisfactory from a morphological point of 

 view, as they do not harmonize with the affinities of the forms of the Arthropods in 

 which the variations occur, neither do they throw any light on the genetic relations of 

 the compound eye. 



Perhaps the following classification will be useful in making the relations of the 

 various forms more easily comprehended. 



1 am so little acquainted with the structure of the eye in the Crustacea from personal 

 observation that I shall confine my remarks chiefly to the conditions which I have 

 observed in the Insecta. 



(I.) Eyes in which the dioptron is incompletely separated from the neuron, each 

 segment of the former having a distinct retinula. I have observed this condition in the 

 larva of the gnats, in Tipula and in the genus Telephorus amongst the Coleoptera. 



