334 bulletin: museum of comparative zoology. 



tions of its function prior to the knowledge of the photokinetic phe- 

 nomenon, has discussed the modern theories about it, and has added 

 new hght on the subject from his own investigations. 



Hesse's anatomical study of the optic organs of many invertebrates 

 yielded two facts bearing upon the present problem: — (1) in certain 

 leeches and annelids are found unpigmented cells which, because of 

 their resemblance to pigmented cells or eye-spots in close relatives, 

 are undoubtedly visual cells; (2) often where pigment is present it is 

 either separated from the sensory cells by an intervening tapetum, 

 as in Pecten and some insects, or it is diversely situated in eyes of 

 closely related species, as among the gastropods. From this it is 

 seen that pigment is not essential to the physiological reception of 

 light nor — as the experiments of Harrington and Learning (:00) 

 and of Mast (:11) on Amoeba indicate,— even of color; and although 

 pigment be present, yet its loose association with the visual cell bars 

 attributing to it a primary role. When the relation between the two 

 is closer, the pigment-cup, by partially screening the cell, enables the 

 animal to determine the direction whence the light comes. 



The phenomenon of migration of the pigment probably arose with 

 the evolution of the eidoscopic eye, with which, at first, only moving 

 objects, and later stationary forms as such, were perceived by the 

 animal. At the outset, before the significance of the pigment can be 

 logically discussed, an answer must be sought to the fundamental 

 question: — where are the receptive organs in the eye of the crayfish 

 located? 



Parker ('95) has emphasized two essential features which the 

 receptive organ must have : — first, a dioptric mechanism for trans- 

 mitting the light to it; and, secondly, nerves for conducting the 

 stimulus to the brain. The phenomenon of the pigment-migration 

 may aid in identifying the receptive region. Thus, Hesse (:00) has 

 questioned Grenacher's conclusion that the rhabdome in the retina 

 of the cephalopod is the sensory organ, and has pointed to another 

 structure as being more probably the sensory organ, because of its 

 closer relation to the emigrated pigment. In the crayfish the optical, 

 neurological, and photokinetic evidence — for a discussion of which 

 see Parker ('95) and Hesse (:01) — points to the rhabdome as the 

 region in question. As to the vertebrate eye, the evidence of neuro- 

 fibrillae on the outer segments of the rods and cones furnished by 

 Hesse (:04) and Howard (:08); the observations by Van Genderen 

 Stort ('87), Chiarini (:04 and :06), and Herzog (:05) of the pigment- 

 migration extending only out to the ellipsoids of the rods; the dis- 



