1890.] OF THE EYE IN ARCTURTJS. 367 



shown to have affected all the component parts of the eye. The 

 cornea was little (S. necera) or hardly at all {S. bromleyana) convex 

 below ; the lens was granular, and could hardly have been transparent 

 during life ; the rhabdom and retinules were not recognizable — 

 at least in the form which they present in other (shallow-water) 

 species. The amount of pigment present was comparatively small, 

 or, as iu iS. bromleyana and S. gracilis, completely absent. I hope 

 to show in the present paper a somewhat similar though less marked 

 series of changes in the eyes of the deep-water Arcturi. 



Before the appearance of my preliminary account of the genus 

 Serolis (1), which contained a summary of observations upon the 

 structure of the eye, but little had been done in investigating the 

 histology of that organ iu deep-sea Crustacea. Dr. P. P. C. Hoek, 

 in his Report on the ' Challenger ' Pycuogonida (6), mentioned that 

 ]iigment is often absent from the eyes of deep-sea forms, and 

 that the retina may be replaced by a mass of connective tissue, 

 though the lens l)e present. The details given by Hoek are not 

 very numerous. Since the publication of my Eeport several other 

 groups of deep-sea animals have been reported on. Mr. S. I. Smith 

 (12) found that in the majority of species of Atlantic deep-sea 

 Decapods the eyes have undergone certain structural changes ; these 

 changes are partly in the alteration of the pigment, which becomes 

 lighter coloured in the abyssal species, and partly in the reduction 

 of the number of the visual elements. 



A considerable number of deep-sea Mollusca according to Pelseneer 

 (8) have rudimentary eyes ; some are totally blind. 



Henderson found (7) with regard to the Anomura that degenera- 

 tion was common in the eyes of abyssal forms : this degeneration 

 was largely shown by the absence or reduction in quantity of the 

 pigment. Here, however, there is no elaboration of detail and the 

 points raised are not illustrated by figures. 



Animals that dwell in caves are, so far as absence of sunlight is 

 concerned, subjected to the same conditions as are deep-sea animals. 

 Packard (10), in investigating animals from the Kentucky caves, 

 found various conditions of degeneration in the eyes, culminating in 

 the total blindness of some species. 



The result, then, of all these investigations has been to show that 

 t/ie deep-sea fauna is chiejiij made up of animals which are either 

 blind or — if they have eyes — shoio evident traces of degeneration in 

 these eyes. 



I attempted to show, iu considering the deep-sea Isopods, that 

 the blind deep-sea genera were, at any rate for the most part, peculiar 

 genera, and that those deep-sea Isopods with apparently well-developed 

 eyes were closely alhcd to, if not identical with, forms living in 

 shallow water. Thus it appeared reasonable to assume that the 

 eyed forms were comparatively recent immigrants into deep water. 

 This view has already, I find, been considered by Prof. Semper ^ to 



^ 'Animal Life,' Int. Scient. Series, p. 84. " We have become acquainted .... 

 with a wonderl'iil deep-sea fauna, showinp the same strikini; mixture of blind 

 and seeing animals a^ the fauna of the (.-aves. This case is all the more 



