PARKER: RETINAL PIGMENT CELLS OF PAL^MONETES. 295 



Four Right Retinas in Light Condition cut off and placed in the 

 Dark about two Hours. 



The four right retinas kept in the light as checks ou the experimeut 

 exhibited the normal condition for the light. 



Although, as the tables show, no case of excised retina with complete 

 photomechanical changes has been observed, several of the cases vrere 

 so nearly complete that I have no hesitancy in stating that, in my 

 opinion, the photomechanical changes in the retina are as little influ- 

 enced by the optic ganglia as by the brain. 



These experiments, then, lead to two conclusions : first, the brain of 

 Palaemonetes is not essential to the complete photomechanical changes 

 of the retinal pigment cells; and, secondly, the optic ganglia are like- 

 wise unessential to these changes. In the latter case, however, the pos- 

 sibility of a slight influence must be admitted. The photomechanical 

 changes of the retinal pigment cells are, in my opinion, induced by the 

 direct influence of the presence or absence of light on these cells. Each 

 cell, then, so far as its mode of action is concerned, is not comparable 

 to a muscle controlled by an efferent nerve, but to a more or less inde- 

 pendent organism, which receives a direct stimulus from the exterioi", 

 and responds appropriately. The uniformity usually shown by the 

 photomechanical movements in the retina as a whole is to be under- 

 stood as an individual but uniform reaction of many separate elements 

 to a uniform stimulus. There is nothing in the action of the retinal 

 pigment cells of Palsemonetes that supports the idea of normal double 

 conduction of nervous impulses. 



General Summart. 



1 . The only parts of the retina in Palasmonetes that exhibit photo- 

 mechanical changes are the three kinds of pigment cells. 



2. The proximal retinular cells contain black pigment granules. In 

 the light these are scattered more or less uniformly throughout the whole 



