ZOOLOGY AND BOTANY. MICEOSCOPY. ETC. 533 



in the sensory epitlielium, wliich is differentiated for the perception of 

 ligLt, sinking into the body and so giving rise to the so-called optic pits ; 

 such organs may be found in Chari/bdea marsiipialis, and in their com- 

 position they differ not at all from the superficial pigment-eyes. Their 

 depression is not only a sign of the more definite localization of the per- 

 cipient spots, but a protective arrangement. Still deeper depression leads 

 to the so-called goblet-eyes, which are first met with in the proximal 

 goblet-eyes of Chari/hdea ; in conjunction with this depression there is 

 the development of a new constituent of the eye — the so-called vitreous 

 body which is differentiated off from the pigment-cells ; it closes the 

 eye cup externally and fills its cavity. A further, though less con- 

 siderable, complication is found in the distal goblet-eyes of Charyhdea, 

 where the invagination of the sensory epithelium forms a secondary 

 outgrowth. In connection with this we meet with a differentiation of 

 the cellular elements which form the wall of the eye ; the cells abutting 

 on the proximal wall of the optic cup are formed of pigment-cells, and 

 this arrangement is functionally that of an iris, for by it the lateral rays 

 of light are cut oft'. 



The goblet-eyes of Aurelia are developed on quite a different type. 

 They are not formed, like the other eyes of jelly-fishes, by the invagina- 

 tion of the sensory epithelium, but of the endoderm and, possibly, of the 

 supporting lamella ; the goblet-wall is, consequently, formed of altered 

 endodermal cells which are filled with pigment. This invagination of 

 the endoderm is followed by that of the optic cells of the ectoderm. As 

 a result of this interesting mode of origin it follows that the nerve-fibres 

 do not arise from inside but from outside the optic cells, and are con- 

 nected with their outer ends ; the free ends appear, therefore, to be 

 turned away from the light, which must pass through the whole of the 

 nerve-layers before it reaches the ends of the optic cells. This relation 

 of parts is not unlike that which obtains in the eyes of Onchidia, Lamel- 

 librauchs, Arachnid-, and Vertebrates. 



The causes, however, are quite different, and the goblet-eyes of 

 Aurelia must therefore be regarded as belonging to a sjiecial type which 

 cannot be directly compared with any yet known type of eye ; there is, 

 indeed, a certain resemblance to the eyes of the Turbellaria. 



The structure of the lens-eyes of Charijhdea, which attain a very 

 high grade of development, is very interesting. They are derived from 

 gublet-eyes, the goblet narrowing at its outer end and becoming con- 

 stricted off from the body-epithelium. The orifice becomes closed, and 

 an optic vesicle completely shut off from the exterior becomes developed ; 

 this is surrounded by the afferent nervous layer as far as its superficial 

 part. In addition to this the body-epithelium grows together at the 

 point where the invagination has taken place, and forms a thin trans- 

 parent layer — the so-called cornea. Simultaneously with this there is a 

 differentiation of the cells which form the outwardly directed wall of 

 the primitive optic vesicle. These increase greatly in length, and, later 

 on, form the spherical lens which projects into the optic vesicle, and 

 occupies a large part of it. The cells of the vesicle at the periphery of 

 the lens become differentiated into the so-called iris. The proximal 

 lens-eye no doubt arises in the same way as the distal, but the proximally 

 directed and not the outer wall of the vesicle becomes differentiated 

 into the lens ; a gelatinous stalk is formed w^hich carries the lens. 



This mode of origin and the peculiarities of these eyes of Charijhdea, 



