PARKER: RETINAL PIGMENT CELLS OF PALASMONETES. 295 
Four Right Retinas in Light Condition cut off and placed in the 
Dark about two Hours. 
Complete change. Partial change. No change. 
Proximal retinular cells 0 2 2 
Distal retinular cells 0 4 0 
Accessory cells . . . . .. 0 3 1 
The four right retinas kept in the light as checks on the experiment 
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 were 
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 
Palæmonetes 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 exterior, 
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 
wo a uniform stimulus. There is nothing in the action of the retinal 
pigment cells of Palemonetes that supports the idea of normal double 
conduction of nervous impulses 
GENERAL SUMMARY. 
1. The only parts of the retina in Palemonetes that exhibit photo- 
mechanical changes are the three kinds of pigment cells. 
2. The proximal retinular cells contain black pigment granules. Jn 
the light these are scattered more or less uniformly throughout the whole 
