MUSEUM OF COMPARATIVE ZOOLOGY. 155 



fully developed eyes the stalk is terminated distally by a hemispherical 

 enlargement ; in the blind crayfishes it ends as a blunt cone. This 

 cone-shaped outline is especially characteristic of C. pellucidus (Fig. 2). 

 It will be observed that in this species the optic nerve (n. opt.) termi- 

 nates in the hypodermis immediately below the blunt apex of the cone. 

 In C. setosus (Fig. 1) the termination of the optic nerve is also at the 

 apex of a blunt cone. In this case, however, the axis of the cone does 

 not coincide with the axis of the stalk, as it does in C. pellucidus, but 

 the two axes meet each other at an angle of about forty-five degrees, 

 and in such directions that the conical protuberance at the distal end of 

 the stalk is directed forward and outward from the median plane of the 

 animal. The protuberance is rather more blunt in C. setosus than in 

 C. pellucidus (compare the regions marked r. in Figs. 1 and 2). 



Through the kindness of Dr. Walter Faxon I was enabled to examine 

 two specimens of C. hamulatus. In this species the stalks also termi- 

 nate in blunt cones. They are not so pointed as in C. pellucidus, but 

 approach the more rounded form of C. setosus. 



The three species, C. pellucidus, C. hamulatus, and C. setosus, are the 

 only blind crayfishes thus far known in North America, and, as they 

 agree in having a conical termination to the optic stalks, a peculiarity 

 not observable in crayfishes with functional eyes, it may be concluded 

 that the conical form is characteristic of the stalks in blind crayfishes. 

 Unquestionably, this conical shape is coupled with the degenerate con- 

 dition of the retina. 



In describing the finer anatomy of the eye it will be more convenient 

 to begin with the condition found in C. setosus. Figure 1 is drawn from 

 a longitudinal horizontal section of the optic stalk in this species. The 

 plane of section passes through the region where the optic nerve and 

 hypodermis are in contact. This region (Fig. 1, r.) corresponds to the 

 retina of other crayfishes. The optic stalk is covered with a cuticula 

 (Fig. 1, ct.), which is of uniform thickness and which resembles the 

 cuticula of the rest of the body. In this respect the stalk differs from 

 that of decapods with well developed eyes, for in these, although much 

 of the stalk is covered with ordinary cuticula, the retinal region is pro- 

 vided with a thin flexible cuticula. This has been named by Patten the 

 corneal cuticula ; it cannot be said to be differentiated in C. setosus. In 

 optic stalks with functional retinas the corneal cuticula is usually 

 facetted, but in C. setosus no indication of facets is discoverable. 



The undifferentiated condition of the cuticula leads one to antici- 

 pate a simple condition in its matrix, the hypodermis. The latter is a 



