PARKER: RETINAL PIGMENT CELLS OF PAL.EMONETES. 277 



optic gaugliou by the retinal uerve fibres. Nerve fibi-es connect the 

 first optic ganglion with the second, the second with the third, and the 

 third with the fourth. From the fourth optic ganglion, which is sit- 

 uated near the proximal end of the stalk, the optic nerve extends to the 

 brain. The finer structure of the optic ganglia in Paltemonetes is in all 

 probability essentially the same as in Astacus, where, as I have already 

 shown (Parker, '95, Taf. 3, Fig. 59), each optic ganglion represents a 

 region of interruption for the great majority of the nerve fibres that 

 intervene between the retina and the brain. 



The retina in Palaemonetes is composed of ommatidia, the structure of 

 which has already been described at length (Parker, '91, p. 108, PI. IX.). 

 For convenience I add a brief summary of this description. Each omma- 

 tidium is composed of five kinds of cells. Immediately under the cor- 

 neal facet (Fig. 1, cm?) are two corneal hypodermal cells {nl. cm.). The 

 distal portion of the axis of the ommatidium is occupied by the cone (con.), 

 which, as seen in transverse sections (Fig. 3, cl. con.), is composed of four 

 parts. Each part contains near its distal end a nucleus (Fig. 1, nl. con.) 

 and represents a cell. The four cone cells are closely applied to one an- 

 other in the region of the cone proper (Fig. 3). Proximally they taper 

 off as thick, more or less independent fibres. (Compare Figs. 1, 4, and 5.) 

 These fibres separate and apparently terminate near the distal end of 

 the rhabdome (Fig. 1, rhh.). I have been unable to trace them further, 

 though I suspected that they might eud, as in Homarus (Parker, '90, 

 p. 14), on the basement membrane. The distal retinular cells either 

 apply themselves to the lower portion of the sides of the cone (Fig. 2, 

 cl. dst.), in which case they are so closely packed that their outlines 

 cannot be distinguished (Fig. 3, cl. dst.), or they occupy a more prox- 

 imal position (Fig. 1, cl. dst.), forming a ring around the attenuated ends 

 of the cone cells (Fig. 5). There is, of course, one ring for each om- 

 matidium. Each ring contains six distal retinular cells, but these rings 

 are so constituted that each cell is at the same time a member of three 

 rings ; hence there are in reality only twice as many distal retinular 

 cells as there are ommatidia. The proximal portion of the axis of the 

 ommatidium is occupied by the rhabdome (Fig. 1, rhb.), which is sur- 

 rounded by seven functional proximal retinular cells (Fig. 6, cl. px.), 

 in addition to which an eighth rudimentary one is present (Parker, '91, 

 p. 111). Each functional cell ends distally iu a somewhat swollen knob 

 containing its nucleus (Fig. 1, nl. px.). From this swollen end the 

 cell extends proximally over the rhabdome, beyond which it becomes 

 slightly attenuated, and, as a retinal nerve fibre (Figs. 1 and 7,/br. r.), 



