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Show 1890.] OF THE EYE IN ARCTURUS. 369 transverse sections (figs. 14-16), of an oblong shape, the corners are sharply marked and the sides are perfectly parallel with, or at right angles to each other. Lower down, at about the level of the nuclei of the retinula-cells (fig. 14), the rhabdom becomes indented, and shows obvious traces of its orgin from six rhabdomeres. Lower down still (fig. 15) the six rhabdomeres diverge from each other. Each rhabdomere becomes surrounded by a dense pigmented sheath. When the eyes are teased in glycerine after depigmentation by nitric acid, the rhabdom shows a tendency to break up into squarish blocks (fig. 8), as has frequently been noticed in other Arthropods. (2) Arcturus spinosus, F. E. Beddard. The eye of this species, which is from deep water, contrasts in many points with that of Arcturus furcatus-a typically shallow-water form. The lens has the peculiar form shown in the drawing (Plate XXXI. fig. 10), which represents a semidiagrammatic longitudinal section through an eye-element. It is somewhat muffin-shaped, being depressed on both sides in the middle. In some other slides which are labelled " Arcturus spinosus," and which I have no reason for doubting are really preparations from this species, the lens has the form shown in another drawing (figs. 6, II); it is pear-shaped, and in the middle it is decidedly more opaque than peripherally, where it is quite transparent. Tbis central opacity may be due to a precipitated and coagulated fluid occupying the interior of the lens, such as Watase (11) has described and figured in Serolis1. I have not, however, observed anything similar in the shallow-water species of Serolis which I myself investigated. Perhaps it will turn out to be a commencing degeneration in the eyes of the species described which is carried out more fully in Serolis neara. The rhabdom of Arcturus spinosus is very large, and in longitudinal sections of the undepigmented eye shows tbe characters exhibited in the drawing (fig. 10) ; it is of roughly conical form, the apex of the cone lying towards the ommateal membrane. In some examples of this species which I referred to above in connection with the peculiar difference in the structure of their lenses, the rhabdom also shows a departure from the ordinary condition. As indicated in fig. 6, its upper extremity embraces the lens, which is sunk into a depression of what is really the broad end of the conical rhabdom ; although in such preparations as those illustrated in figs. 6, 11, the vitreous body and the rhabdom appear to be very nearly if not quite in actual contact, there is not the least difficulty in distinguishing between them. The rhabdom in both forms of eye is by no means so clear and transparent as in Arcturus furcatus, and it is proportionately very much larger than in that species. Its form varies much, but is usually more or less bent. 1 Loc. cit. p. 290, pi. xxix. fig. 1 a. |
