1890.] OP 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. 1-1), 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 np 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 Arctu7'us furcatus — a typically shallow- 

 water form. 



The lens has the peculiar form shown in the drawing (Plate 

 XXXI. fig. 1 0), which represents a semidiagraramatic longitudinal 

 section through an eye-element. It is somewhat muffiii-sliaped, 

 being depressed on both sides in the middle. In some other slides 

 which are labelled '' Arctwus 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, 11); it is pear-shaped, 

 and in the middle it is decidedly more opaque than peripherally, 

 where it is quite transparent. This 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 Serolis^. 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 necera. 



The rhabdom of Arcturus spinosus is very large, and in longi- 

 tudinal sections of the undepigmented eye shows the characters ex- 

 hibited 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, 1 1, 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 /urcatus, and it is proportionately very 

 much larger than in that species. Its form varies much, but is 

 usually more or less bent. 



1 Loc. eit. p. 290, pi. xxix. fig. 1 a. 



