OCR Text |
Show 370 MR. F. E. BEDDARD ON THE STRUCTURE [May 6, The retinula-cells are, on the other hand, very much smaller proportionally, and are only well developed and conspicuous at the end of the rhabdom, which, 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 Arcturus furcatus. The pigment, although deep black in colour, is very much less in amount than it is in A. furcatus. 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 preparations 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 with that of the ommatidium when the lens is in position. The lens agrees, however, with that oi Arcturus 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 correspond with the cells of the retinula. The retinula-cells themselves, as in A. spinosus, 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 attention 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 |