370 MR. F. E. BKDDARU ON THE STRUCTURE [May 6, 



The retinula-cells are, on the other hand, very much smaller pro- 

 portionally, and are only well developed and conspicuous at the end 

 of the rhabdom, whicli, by the way, shows no traces of division into 

 six rhabdomeres. 



As may be seen by longitudinal sections (fig. 10), a coating of 

 dense pigment covers the rhabdom, and occasionally pierces into its 

 interior for a short distance ; this I presume to be the upper portion 

 of the retinula-cells. The nuclei of these cells are placed on a level 

 with the posterior end of the rhabdom instead of near the upper 

 extremity of this structure, as they are in Arcturua farcatas. The 

 pigment, although deep black in colour, is very much less in amount 

 than it is in A. fiircatus. It is clear, therefore, that the eye of 

 Arcturus furcatus differs from that of A. spinosus in many points. 



I do not possess so many prefiarations of other species of Arcturus 

 as of the two which I have just described. The following notes 

 therefore show many lacunae which I see no chance of being able to 

 fill up. They are largely but not entirely based upon sketches 

 which were made some five years ago, when I commenced to work 

 at this subject. These sketches, unfortunately, do not show all the 

 points which I have since ascertained to be important. 



(3) Arcturus anna, F. E. Beddard. 



In this species the lens has an ellipsoidal form, the long axis coin- 

 ciding with that of the ommatidium when the lens is in position. 



The lens agrees, however, with that of ^rcf writs spinosus — at least 

 with some individuals of that species — in being composed of a clearer 

 peripheral portion and a granular-looking opaque middle. 



The rhabdom is large and solid, it is not prolonged into six 

 separate rhabdomeres at the posterior extremity as in A. furcatus ; 

 after treatment with nitric acid, however, it shows distinct traces of 

 longitudinal division into a number of pieces which no doubt corre- 

 spond with the cells of the retinula. 



The retinula-cells themselves, as in ^. sjomostts, are only clearly 

 distinguishable as such behind the rhabdom where their nuclei are 

 situated ; a coating of pigment which covers the rhabdom up to very 

 nearly the lens is doubtless deposited in a forward prolongation of 

 the retinula-cells. 



(4) Arcturus cornutus, F. E. Beddard. 



In most respects the eye of this species agrees with that of 

 Arcturus anna ; so close is this agreement, that I need not enter into 

 any description of the ommatidium. All that I shall do is to call atten- 

 tion to one rather important point of difference between this species 

 and Arcturus anna. 



This point of difference concerns the vitreous body, which appears 

 to be even less fitted as a refracting medium in this species than in 

 the last. 



The opacity, which is quite a noticeable feature of the lens in 

 A. anna, is exaggerated in A. cornutus, until there is not even a 



