1890.] OF THE EYE IN ARCIURUS. 3/3 



hand, in teased preparations of A. furcatus the rhabJom 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 necera ; 

 in that species (a deep-sea form) the pigment is just as deuselv black 

 as in the shallow-water Serolis cornuta, but less in amount'. 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 of 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. fur- 

 catus and A. americamts. 



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 o( Arcturus. 



These differences fall into two main categories : — 



(1) In A. furcatus and A. americanus (?) the rhabdom is com- 

 paratively 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. glnci- 

 alis, and A. studeri the rhabdom is veiy lar^e and the retinula-cells 

 are comparatively small, their nuclei^ being situated below the ex- 

 tremity 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 iu 

 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. Beddard, F. E. — Preliminary Notice of Isopoda collected 



' 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 retiuula. 



'^ 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 



