VISUAL TISSUES 233 



visual cells. The perceptory organ is separated from the cell body by 

 a cytoplasmic process instead of resting directly upon it. The afferent 

 process is thus almost a fiber, like the efferent process. 



All these planarian eyes have sunk below the surface, and the break 

 in the outer epithelium has been grown over by its simple cubical cells, 

 which thus form a cornea, together with the connective- tissue elements 

 between them and the eye. But it is an unspecialized cornea and not 

 different in any way from the rest of the integument. There is no lens 

 present. 



Another eye type of which space forbids a full description is that seen 

 in the leech. Here also we meet with an organ that contains from one 

 to upward of forty visual cells. The degree of specialization is a high 

 unicellular one. The visual cell-organ is peculiar, and contains in its 

 body both retinal and lens structures. 



Going back to the echinoderms, in which we found the simplest type 

 of eye in a starfish, we find our next step of development materialized 

 in one of the urchins, Diadema, where the eyes are developed on the 

 bases of the spines. 



This eye is somewhat compound ; that is, it is composed of a number 

 of units, any one of which would represent an efficient organ of light 

 perception. These units are each composed of several different cell 

 groups or tissues, one of which, the superficial cornea, is common to 

 all of them. Each unit is an upright, five- or six-sided column whose 

 proximal end is somewhat pointed and rests in a pigment cup. This 

 pigment cup is composed of mesodermal cells which have moved up 

 from below and formed the pit-like cavity which opens distally. The 

 cup forms the sides of the eye-unit or ocellus. 



The larger body of each ocellus is an oval mass, made up of ten or 

 twelve large transparent and refractive cells with small nuclei. These 

 cells are elongate and roughly wedge-shaped with the sharp ends pointed 

 toward each other and interlocking, while the blunt ends rest on the sur- 

 face of the mass. This organ is undoubtedly the lens, as can be told 

 by its transparent and refractive nature and the position which it occu- 

 pies. 



The lens is capped distally and proximally by two caps, each com- 

 posed of a single layer of cuboidal cells and each covering nearly a 

 third of the lens surface. These caps are well shown in two middle 

 ocelli, where they are represented as in a surface view (Fig. 206). In 

 the ocelli, to right and left, they are seen in median section only. They 

 are much alike, and only the position seems to decide that the proximal 

 cap must be composed of visual or light-perceiving cells. The upper 

 cap has been spoken of by the Saracens as a germinative group of cells 

 from which the lens is formed and removed. 



