98 



3. Total atrophy of the optic lobes, optic nerves, and all the optic 

 elements. 



The degeneration of the eye of the species under consideration has evi- 

 dently folloAved the second of these lines. The optic stalk has suffered 

 a foreshorti'uin.u-. and as a. cnnseijucnice the optic sangiia liaA'e become 

 telescoped. The greatest reduction lias taken place in tlie ecto-dernial por- 

 tions of the eye. which are reduced to ;i group of cells not exceeding and 

 probably fewer than '■>'>(>. Inasmuch as a single normal ommatidium con- 

 tains sixteen cells, the degree of degeneration reached is readily seen to be 

 very great. 



The extent of Hie nioditicat ion of the eyes can i)erliai)S be most ri'adily 

 dcsci-ilied l)y ;i (•innii.irisoii of the eyes and optic stalk' of this species witli 

 those of r.-iliiMiionetes exiliiies. taken in Hie S:ni .Marcos Itiver. lint a sliort 

 distauce from tlie artesian well. 



The eye and oi>tic st.-ilk of V. exilipes iireseiits tlie general appear- 

 ance of the crustacean eye. Tlie stalk is a tniiiciite cone (Fig. 1), at- 

 taclied liy its siiiaber end. On tlie distal end is tlie lai'ge. dark, consiiicn- 

 oiis. hemisiphei-ic;il eye. It is \\ ider tlian tlie widest part of tlu' stalk. 



In I'. ;iiitro]Mim the eye stalk is niucli smaller ( I'Mg. "Ji, .as may be seen 

 from the following l.-ihle: 



exilipes. antrorum. 



Length of specimen 17 mm. 15 mm. 



Length of stalk to retina 787 // 525 /^ 



Width of stalk at retina 700 // 175 // 



Width of stalk at base 387 ," 337 /-t 



Width of retina 962.5 // 



Nothing apiieais to remain of the eyes excejit llie short, colorh^ss, 

 delicate stalks. The stalks ;ire conical, being .-ittJiched by their larger 

 end. The axis of tlH> stalk is liamllel with that of the body. 



The distal end of the optic stalk of P. antrorum is covered with a single 

 layer of indifferent hypoderm with nuclei 7.2 m by 3.6/v, except at a short dis- 

 tance from the distal end of the outer lower quarter of the stalk, where a 

 group of slightly modified hypodermal cells, three deep, replace the single 

 series of outer parts. The nuclei in this group of cells are rounded, measur- 

 ing about 6 to 8 // in diameter. This group of cells measures about 50 /a by 

 70//. There is no indication of an arrangement of the.se cells into anything 

 resembling the arrangement of the cells in an ommatidium. 



