MUSEUM OF COMPARATIVE ZOOLOGY. 161 



other crayfishes presented the same condition as that shown in Figure 2. 

 In the specimen 5.6 cm. in length, the granular bodies were less dis- 

 tinct than in the other two, but they were nevertheless recognizable, 

 and the retinal thickening was as pronounced in this as in either of the 

 other specimens. The fact that these three individuals show so little 

 variation leads me to believe that the condition of the eye in the blind 

 crayfish is not so variable as I at first supposed it would be. The same 

 constancy is also true of C. setosus. Hence it seems to me improbable 

 that the differences between Newport's observation and those of the 

 later investigators are due to individual variations in the specimens 

 studied. The fact that Newport's work was done before the develop- 

 ment of present methods of research offers, I believe, a more natural 

 explanation of some of his results, than the supposition of individual 

 variations. That the methods of his time were imperfect is evident 

 from the fact that Newport himself seems to have overlooked the gan- 

 glion of the optic stalk, a structure readily discoverable by means of serial 

 sections. (Compare Newport's Figure 13 ['55, p. 102] with Figure 2 in 

 this paper.) Ley dig's observations, so far as they extend, are fully con- 

 firmed by my own. Packard's account differs from mine in only one par- 

 ticular, but that is of considerable importance ; he states that there is 

 no retinal thickening in the two species studied by him. This difference 

 may possibly be due to individual variations in the crayfishes. Unfor- 

 tunately, Packard does not state the number of specimens which he 

 examined, and consequently one is uncertain how much weight to give 

 to his general statements. 



The conclusions to be drawn from the foregoing account may be 

 summarized as follows. In both species of crayfishes studied, the optic 

 ganglion and nerve are present, and the latter terminates in some way 

 not discoverable in the hypodermis of the retinal region. In C. setosus 

 this region is represented only by undifferentiated hypodermis, com- 

 posed of somewhat crowded cells, while in C. pellucidus it has the form 

 of a lenticular thickening of the hypodermis, in which there exist multi- 

 nuclear granulated bodies. These I have endeavored to show are 

 degenerated clusters of cone-cells. If Packard's observations are correct, 

 the retina in C. pellucidus may be reduced in some individuals as much 

 as it is in C. setosus, which I have studied, but my own examinations 

 do not render this view probable. 



Cambridge, February 24, 1890. 



