454 MKMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



Tlie proliferating areas iu the optic disks of Alpheus ami Homarus tiro undoubtedly homologous, 

 and probably correspond also to tlie optic invaginations described in Astacus by Reicheubacb aud 

 in Crangon by Kingsley. I therefore agree with Parker (47) in interpreting the ingrowth or invo- 

 lution of ectodei in, whichever may occur in the developing disk, as concerned with the optic gan- 

 glion solely and not with the retina. Reicheubach describes the. visual organs as originating from 

 three factors: (1) an epidermal layer; (2) the optic invagiuation ; (3) the optic or segrneutal 

 ganglion. From the epidermal layer and outer wall of the optic iuvagiuatiou the retina arises, 

 while the inner wall of the secondary iuvagiuation unites with the optic ganglion. An inspection 

 of ISeichen bach's Fig. 224 (54) shows, as Parker has pointed out, that in all probability Reiehen- 

 bach has misinterpreted his sections, and that the entire retina is derived from the hypodermal 

 layer. The layer of cells with elongated nuclei, which he has designated as rhabdom, clearly per- 

 tains to the optic ganglion, and probably represents the nuclear covering of the distal convex 

 surface of the lame gangliouaire (Fig. 192 of this work). 



Kiugsley, iu his third paper on Crangon, changes his interpretation of the imagination of the 

 optic disk of Crangon, regarding this involution as concerned only with the optic ganglion. I am 

 inclined to believe that a renewed study of this subject would show that the optic disk originates 

 iu Craugon precisely as it does iu Alpheus. A series of sections through the thickening disk of 

 Crangou has little to show which is not brought out by a similar series of Alpheus (Figs. 76-83), 

 and no pit or hollow imagination is seen. 



The independent origin of the optic ganglia lends some support to the view that they have a 

 segmental value and are not merely outgrowths from the brain, that the eyestalk is a modified 

 appendage containing its proper ganglia. 



Watase's interesting views (63) concerning the origin of the ommatidium from a hypodermal 

 pit do not receive the support we should expect from embryology. How much value is to be given 

 to the embryological data iu this case it is hard to say, but, seeing the persistence of the involu- 

 tions in the eye of Limulus, we would expect to find a trace of similar iufoldings in the developing 

 eye of the lower Crustacea, provided their eyes are constructed upon the same type. Until greater 

 evidence is furnished I am inclined to regard the "compound eye" not as an aggregate of simple 

 eyes, as the name implies, each one of which is due to a hypodermal infolding, but rather, as Par- 

 ker has suggested, to differentiated clusters of ectoderm cells originating from a single epithelial 

 layer. 



THK KYE CNDEK THE INFLUENCE OF LIGHT ANIi 1>A l:KX!:s.-. 



Ill June, 1890, while enjoying the facilities for biological research afforded by the labora- 

 tory of the U. S. Fish Commission at Woods Holl, Mass., it occurred to me that some valuable 

 experiments could be made by testing the effects of direct sunlight and total darkness upon the 

 growth aud behavior of the pigment cells of the compound eye of Crustacea. After finishing my 

 experiments upon one form I learned of the experimental work of Exuer* upon the eyes of the 

 glowworm, Lampyris npUndidula , of Hydrophilus, Dysticus, and Colymbetes, in which he records 

 the same phenomenon in insects which I have observed in a Crustacean. Later a paper has also 

 appeared, by Mademoiselle M. Stephauowska, on the histological arrangement of pigment in the 

 eye of Arthropods. I have seen only an abstract of this work, from which I gather that it deals 

 either largely or entirely with the eyes of insects. My experiments were made upon the common 

 prawn, Palcvmonetes rulgaris. 



A dark chamber was constructed aud rendered as absolutely light-proof as possible. Inside 

 of this a small glass aquarium was so arranged that a stream of sea water could be kept run- 

 ning through it for any length of time. Three egg-bearing females were then placed in the 

 aquarium and the chamber was sealed. The egg embryos were early nauplius stages. Females 

 with eggs iu a similar stage were also kept under observation iu an aquarium exposed to the light. 

 The general cast of color of the prawn taken in the light is some shade of light brown or brownish 

 green. After spending eighteen days in the dark, the prawns were taken out aud exposed to tln> 

 moderately bright light of the laboratory. The eyes were jet black aud appeared to nave greatly 



Since these notes were written I have received the completed work of Exner, Die Physiologic (let- FacMirten 

 ran lirel>*tii iniil Iimtcleii. in which the field of experiment is greatly enlarged. 



