294 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
second table the photomechanical changes were carried out completely, thus 
demonstrating that the brain is not in any way essential to these changes. 
Why this completeness was not seen in other cases I am unable to state 
positively, though I believe it was owing to the changes that gradually 
appear in the tissue of the stalk after its severance from the body of the 
animal. When an optic stalk is excised, the blood in it soon coagulates 
and other alterations doubtless start up, which finally result in the com- 
plete death of the tissues of the stalk. It is these alterations, I believe, 
that overtake and bring to a standstill the slowly progressing photome- 
chanical movements. But, whatever may be the true explanation of the 
incompleteness of the changes in excised stalks, the general conclusion 
remains unaffected, that in Palaemonetes the brain is not essential to the 
photomechanical changes in the retina. 
This conclusion has an important bearing on the question of the sym- 
pathetie relations of the two retinas in a given animal. Since the two 
retinas are nervously connected only through the brain, and since the 
retinas are not influenced from the brain, it follows that the two retinas 
cannot be sympathetically related, a conclusion to which observations 
already recorded have likewise pointed. 
If the photomechanical changes are not dependent in any degree on 
the brain, it may still be asked whether they are not influenced by the 
optic ganglia. To answer this question, I carried out on excised retinas 
a series of experiments similar to those just described for the optic 
stalks. It is much more difficult to separate the retina from the optic 
ganglia than it is to separate the optic stalk from the brain, but with 
careful manipulation it can be done, and the following tables give the 
results of experiments carried out upon such retinas. 
Four Right Retinas in Dark Condition cut off and placed in the 
Light about two Hours. 
Cqmplete change. Partial change. No change. 
Proximal retinular cells. 0 4 0 
Distal retinular cells . . . - 0 3 1 
Accessory cells 0 4 0 
The four left retinas kept in the dark as checks on the experiment 
exhibited the normal condition for the dark. 
