232 



HISTOLOGY 



epithelial surface from which it originated and lies within the body tis- 

 sues, one of whose cells has moved alongside of it and developed black 

 pigment in some of its cytoplasm. The pigmented portion of the 

 body is arranged in a crescentic form, with the nucleus placed on the 

 outer side in a small portion of undifferentiated cytoplasm. 



The distal cytoplasm of the visual cell is much enlarged, and forms 

 a mass that is bent laterally and lies in the concave side of the pigment 

 cell. The distal edge of this mass is modified into a denser substance, 

 the cell-organ of sight or rhabdome, which forms a lining against the 

 interior of the cup-shaped pigment cell. 



It should be noticed here that this visual cell is inverted; that the 

 rays of light must pass through the cell body and nucleus as they enter 

 the opening of the pigment cup and strike the rhabdome from its proxi- 

 mal side. The efferent pole of the visual nerve cell is drawn out into a 

 fiber that passes inward, to some ganglion where its impulse can be 

 discharged and used. 



Concerning the central connections of this fiber but little is known. 

 It can be said, however, that it forms a very simple pathway for 

 the light-perception impulse, the greater part of the distance being 

 furnished by the one drawn-out process of the visual cell itself. This 

 is not true in any eye that forms an image. 



The eyes of other planarians 

 are often but larger forms of 

 this monocellular organ with its 

 monocellular pigment cup. In 

 the eye of Amandia (Fig. 204, 

 J5) there are three or five visual 

 processes from the one cell, with 

 the perceptory organs on the 

 ends of the processes. These 

 processes are directed into the 

 same kind of a mononuclear 

 pigment cup. 



In Planaria gonocephala we 

 find the same kind of an organ, 

 except that there are from 

 twenty to thirty or more visual 

 cells, and the pigment cup is 

 made of a layer of over a hun- 

 dred separate cells instead of 

 only one cell. As in the unicellular form of pigment cup, the nucleus 

 is always in the proximal part of the cell body and the pigment is in 

 the distal part (Fig. 205). A slight difference is to be seen also in the 



ms. r. 



FIG. 205. Axial section of the eye of a planarian 

 worm, Planaria gonocephala. vis.c., visual cells; 

 vis.r., visual rods or rhabdomes; nv.f., centripetal 

 fibers of visual cells; pg.c., pigment cells. (After 

 R. HESSE in Zeits.f. wiss. Zool.) 



