ZOOLOGY AND BOTANT, MICROSCOPY, ETC. 725 



only found around the orifice of the cup ; the retina is almost 

 diagrammatically simple in construction. There is a lens, which is 

 formed of a gelatinous substance. On treatment with reagents 

 (chromic acid), it breaks up into radially arranged fibres. The 

 characters of the optic nerve are somewhat remarkable, for it breaks 

 up into two or three branches before entering the eye ; the nerve- 

 branches then broaden out in such a way as to enclose the whole eye 

 and to come into direct connection with the retinal cells. In the peri- 

 pheral portion of the optic nerve it is only possible to make out 

 a large number of small ganglionic cells, but in the enlargements 

 which surround the bulb there are cells which are three times as large 

 as these. The eye is placed in a mass of loose connective tissue, and 

 there is no indication of a sclerotic or of any other investing 

 membranes. 



The eye of Fusurella affords, according to Dr. Fraisse, an example 

 of how very different in structure the same parts may be in closely 

 allied animals. The eye of F. graeca lies just below the epidermis, 

 the corneal being separated from the epithelial cells by a delicate 

 layer of connective tissue. As a rule the eye is well rounded ; though 

 there is not here the same close connection as in the two previous cases 

 between the epidermal and retinal cells, there is no doubt that the 

 latter are derived from the former. These retinal cells belong to two 

 groups ; they may be long and very delicate at their lower ends, or 

 they may be broader and thicker and more coarsely granulated, and 

 the latter are, moreover, devoid of pigment. These thicker basal 

 cells are regarded by the author, not as supporting-cells of the true 

 retinal elements, but as those organs from which the lens and vitreous 

 body are developed ; the pigmented cells alone function as the end- 

 organs of the optic nerve. 



In conclusion, the author points out that if we compare the above 

 described sensory organs with one another and with those of other 

 animals, we shall find that the eye of Patella presents the very 

 simplest condition, giving an indication of the phylogeny of the eye. 

 The nearest ally of Patella is CMton, which in its adult, though not 

 in its young stage, is eyeless. In Nautilus the eye is likewise open 

 to the exterior, and in the Hirudinea we may find organs of a some- 

 what similar construction. In cases of this kind the use of the term 

 retina should be avoided, and be replaced by that of rod-cells. The 

 cells which appear to be the organs for the perception of light are very 

 characteristically developed in the MoUusca ; what is here seen almost 

 in diagram is found more or less distinctly in all other Molluscs. The 

 so-called retina consists of a series of elongated cells, the anterior 

 portion of which is filled up by dark pigment. This pigment is more 

 or less marginal in position, so that there is in the centre an unpig- 

 mented cylindrical canal which passes directly into the unpigmented 

 part of the cell. 



Finally, it is suggested that a comparison should be instituted 

 between these bodies and the rods in the eyes of the Arthropoda. 



