MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 455 



swelled in size, and the body was bleached nearly white. The peculiar appearance of the eyes ICHS 

 caused !>// the for/card extension of the distal retinular cells, of which there is a single pair in ench 

 ommatidium. 



The eggs of some of the prawus were hatching, and the pigment of the /ora was carefully 

 compared with that of the first larva of Palremouetes hatched in the light. Roth the black pigment 

 of the retinnlar cells and the yellowish green pigment of the accessory pigment cells of the eye 

 and the large brown chromatophores in different parts of the body were of the same character, 

 whether the embryo had developed in darkness or light. 



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

 sented the same appearance. As in the other cases, as soon as light reached the eye the distal 

 retinula? began to retreat to a deeper level. At first the black pigment which characterizes these 

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

 transparent band is seen below the cornea. This light zone increases as the pigment continues its 

 retreat until, in the course of three-quarters of an hour, the distal retinula? eusheath only the 

 lower ends of the cones. 



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

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

 light of the room twenty-five minutes. The distal retinular cells thus respond very promptly to 

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

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



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

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

 ish brown pigment belonging to the accessory pigment cells; a wider and much lighter area 

 peppered with dark granules, pertaining to the proximal retinular cells, the nuclei of which form 

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

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

 retiuular cells. As stated above, there are two of these cells to each ommatidium, and they each 

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

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

 tion of the 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. The distal retiuular cells thus sur- 

 round the proximal ends of the cone cells. 



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

 about midway between the corueal cuticula and the layer of nuclei of the proximal retinular cells. 

 The nuclei occupy a central position in this layer. Piginented pseudopodia extend forward to the 

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

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

 edly have occupied a still more peripheral position. 



In the eye kept in the darkness for the same length of time and afterwards exposed to the 

 light for five hours the distal retiuular 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 retiu= 

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

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

 by light the ribbon unfolds as the cell travels forward. 



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

 retinal image in 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 principal embryo- 

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

 interesting results. 



