MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 455 



ssvelled in size, iind the liody was bleached nearly white. The peculiar appearance of the eyes irax 

 cauficd 1)1/ the I'orirard extension of the distal retinidar cells, of which there is a single pair in each 

 ommatidium. 



The e^ius of some of tiie inawiis were batching, and the pij^ment of the zoea was carefully 

 compared with that of the tirst larva of Paheuiouetes batched in the li^ht. IJolli the black pigment 

 of the letiniilar cells and the yellowisb green pigment of tbe accessory pigment cells of the eye 

 and tbe large brown cbromatopbores in different i)arts of tbe body were of tbe same cbai-acter, 

 whether tbe embryo bad develoi)ed in darkness or light. 



Another prawn was kept in tbe dark thirty-eight days, and on exposure to the light it pre- 

 sented the same ai)pearance. As in the other cases, as soon as light reached the eye tbe distal 

 retiuube began to retreat to a deeper level. At tirst the black i)igment which characterizes these 

 cells extends out to the cornea. After an exposure of two minutes to direct sunlight a slight 

 transparent baud is seen below tbe cornea. This light zone increases as the i)igment continues its 

 retreat until, in tbe course of three-quarters of an hour, tbe distal retinubu ensbeatb only the 

 lower ends of tbe cones. 



In another experiment a prawn was left only about twenty-four hours in darkness. Tbe same 

 effects were produced in tbe eye, which assumed its former condition after being iu tbe diffused 

 light of tbe room twenty-tive minutes. Tbe distal retinular cells thus respond very ))romptly to 

 tbe action of tbe light, and in tbe course of a few hours (tbe exact time needed was not determined), 

 if excluded from tbe light, completely enshroud tbe proximal ends of tbe cone cells. 



In the eye of Pahemonetes, taken in ordinary daylight, there are three distinctly marked strata 

 of pigment between tbe basement membrane and the cornea, a proximal narrow stratum of yellow- 

 ish brown i)igment belonging to tbe accessory pigment cells; a wider and mucb lighter area 

 peppered with dark granules, pertaining to tbe proximal retinular cells, tbe nuclei of wbicb form 

 a conspicuous belt or layer on a level with tbe distal extremity of tbe rhabdoms. Lying close 

 upon the tier of retinular nuclei is a thin stratum of intensely black pigment, composed of the distal 

 retinular cells. As stated above, there are two of these cells to each ommatidiun), and they each 

 send out a slender thread-like process, which extends in some cases as far forward as tbe corneal 

 cuticula, where it is ])ossibly attached. I have not detected any similar prolongations in the direc- 

 tion of tbe basement membrane. Below the level of the cone, which terminates abruptly in a 

 convex proximal surface, the cone cells are prolonged into a long slender stalk consisting of proto- 

 plasm or of a refractive substance of a different nature from the cone. The cone cells are not 

 apparently prolonged below the level of the retinular nuclei. Tbe distal retinular cells thus sur- 

 round the proximal ends of the cone cells. 



In tbe eye exposed for thirty-eight days iu the dark tbe distal retinular cells form a stratum 

 about midway between tbe corneal cuticula and tbe layer of nuclei of tbe proximal retinular cells. 

 The nuclei occupy a central position in this layer. Pigmented pseudopodia extend forward to tbe 

 cornea, and occasionally a cell shows a slight inward prolongation. Had the eyes been preserved 

 without bringing them into the light, even for a moment, tbe distal retinular cells would undoubt- 

 edly have occupied a still more peripheral position. 



In tbe eye kei)t in the darkness for tbe same length of time and afterwards exposed to the 

 light for live hours the distal retinular cells have retreated until they lie around the proximal 

 ends of the cones. The nuclei of these cells lie immediately upon the nuclei of the proximal retin- 

 ular cells, and it is interesting to notice tbe pigmented body of each cell folded on itself. In 

 section tbe pigment takes tbe form of plaited black ribbons. When tbe eye is again stimulated 

 by light tbe ribbon unfolds as the cell travels forward. 



These cells are called by Exner tbe iris pigment, since they regulate the brightness of the 

 retinal image iu much the same way as the vertebrate iris does. 



X. — SUMMARY. 



In the review, including Sections v-ix of Part Second of this memoir, the i)rincipal embryo- 

 logical facts have been summarized, and it will now suflBce to recapitulate only some of tbe more 

 interesting results. 



