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 followed the second of these lines. The optic stalk has suffered 

 a foreshortening, and as a consequence the optic ganglia have become 

 telescoped. The greatest reduction has taken place in the ecto-dermal por- 

 tions of the eye, which are reduced to a group of cells not exceeding and 

 probal>ly fewer than o50. Inasmuch as a single normal onimatidiuu) con- 

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

 very great. 



The extent of the modification of the eyes can perhaps be most readily 

 described by a comparison of the eyes and optic stalk of this species with 

 those of Palsemonetes exilipes. taken in the San Marcos River, but a short 

 distance from the artesian well. 



The eye and optic stalk of P. exilipes presents the general appe^ar- 

 anee of the crustacean eye. The stalk is a truncate cone (Fig. 1), at- 

 tached by its smaller end. On the distal end is the large, dark, conspicii- 

 ous, hemispherical eye. It is wider tlian the widest part of the stalk. 



In P. antrorum the eye stalk is much smaller (Fig. 2». as may be seen 

 from the following table: 



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// 



Width of retina 962.5 n 



Nothing appears to remain of the eyes except the short, colorless, 

 delicate stalks. The stalks are conical, being attached )>y tlieir larger 

 end. The axis of the stalk is parallel with that of tlie body. 



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

 layer of indifferent hypoderm with nuclei 7.2// by 3.6 ,", 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 groiip of cells measures about 50 // by 

 70/x. There is no indication of an arrangement of these cells into anything 

 resembling the arrangement of the cells in an ommatidium. 



