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Show 1890.] OF THE EYE IX ARCl'URUS. 373 hand, in teased preparations of A. furcatus the rhabdom always 'appeared as a densely black mass in the centre of the retinula-cells, its outline only being recognizable ; although in these deep-sea forms the amount of the pigment is very decidedly less than that which is found in the shallow-water species A. furcatus, its colour is the same; in all forms it had a dense black appearance. These facts are similar to those which I stated with reference to Serolis neara ; in that species (a deep-sea form) the pigment is just as densely black as in the shallow-water Serolis cornuta, but less in amount1. On the other hand, it has been several times observed that in other deep-sea Crustacea the pigment is of an orange colour. This I suppose only means that the pigment-granules are less dense in those forms ; for in the species of Arcturus which I describe in the present paper the pigment when dissolved by means of nitric acid showed an orange-brown colour. Unfortunately I am not able to state what is the amount of pigment, as compared with other forms, present in the ommatidia oi Arcturus studeri. It agrees, as I have pointed out, with other deep-water forms in the small size of the retinula-cells and in the position of their nuclei below the level of the extremity of the rhabdom, but it has a large clear vitreous body like that which is found in each ommatidium of the eye of A. furcatus and A. americanus. In any case I have been able to describe in this paper for the first time certain interesting differences of structure in the eyes of a number of species of Arcturus. These differences fall into two main categories:- (1) In A. furcatus and A. americanus (?) the rhabdom is comparatively small (though large compared with other Crustacea), and the retinula-cells are very large, the nuclei being situated at the level of the anterior end of the rhabdom. (2) In A. spinosus, A. anna, A. cornutus, A. brunneus, A. glaci-alis, and A. studeri the rhabdom is very lar^e and the retinula-cells are comparatively small, their nuclei2 being situated below the extremity of the rhabdom, near to the basement membrane of the ommateum. Besides these morphological differences in the retinula-cells, which perhaps have no reference to the conditions under which the animals live, the second division shows various peculiarities in most species which seem to be correlated with a deep-sea habit. Thus in some forms the lens is reduced in size and altered in form or has become partially opaque and the pigment is small in amount; these statements apply to all the species in the second list except Arcturus studeri. List of Memoirs referred to. 1. B E D D A R D , F. E.-Preliminary Notice of Isopoda collected 1 It will be remembered that in the case of this deep-sea Serolis the small amount of pigment is also correlated with degeneration of the retinula. 2 This position of the nuclei, though unusual, is not unparallelled. They occur in that position in Talorchestia, even below the ommateal membrane (Watase), and in other Amphipods. PROC. ZOOL. SOC-1890, No. XXVI. 26 |
