278 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



pierces the basement membrane {mb. ba.) and extends to the fii-st optic 

 gaugUon. Here it probably terminates in a fibrillation, as has already 

 been shown to be the case in Astacus (Parker, '95, p. 41). The acces- 

 sory pigment cells (Fig. 1, cL sn.) occupy the space in the deeper part 

 of the retina. The number of these cells is not constant, but, judging 

 from their nuclei, it is not more than one or two for each ommatidium. 

 Proximal processes extend from these cells through the apertures in the 

 basement membrane to the distal surface of the first optic ganglion, and 

 distal processes may extend forward to the front faces of the distal 

 retinular cells. Each ommatidium in Palsemonetes, then, is composed 

 of the following cells : two corneal hypodermal cells, four cone cells, two 

 distal retinular cells, eight proximal retinular cells (one of which is 

 rudimentary), and a variable but small number of accessory pigment, 

 cells. Black pigment granules are contained in both the distal and the 

 proximal retinular cells, and are limited to these cells ; the whitish pig- 

 ment lies exclusively in the accessory pigment cells. The seven func- 

 tional proximal retinular cells are the only elements of the ommatidium 

 that are known to have nervous connections. These brief anatomical 

 statements may suffice as an introduction to the consideration of the 

 pigment changes in the retina. 



PHOTOTreCHANICAL ChAXGES IN NORMAL PiETIXA. 



The general method by which the normal photomechanical action of 

 the retinal pigment cells in Palsemonetes was determined consisted in 

 the examination of eyes that had been kept in the light or in the dark 

 known periods of time. For a dark chamber I used a box with a tight- 

 fitting cover. Fi-om time to time during tlie course of the experiments 

 this box was tested for its light- proqf qualities by exposing in it a very 

 sensitive bromide paper, such as is used by photogi-aphers. In all my 

 experiments this showed complete absence of light. The top of the 

 box was pierced by a hole, through whicli a piece of rubber tubing was 

 introduced so that fluids could be poured into the box without exposing 

 its contents to light. Two or three turns in this tube were found suffi- 

 cient to pi-event such light as entered the outer end of the tube from 

 reaching the interior of the box. Living shrimps in a vessel of water 

 were placed in the box, and the cover was carefully closed. After the 

 expiration of the required interval, hot water was run in through the 

 tube, and the animals were thus killed in the dark. Other killing re- 

 agents, such as corrosive sublimate, picric acid, etc., were tried, but 



