MUSEUM OF COMPARATIVE ZOOLOGY. 157 



same condition. Figure 1 is taken from one of the larger individuals. 

 In this specimen the cuticula was somewhat thinner and the hypoder- 

 mis rather thicker than in the other two. This I believe was due 

 to the fact that the animal had recently moulted. 



So far, then, as the eye of C. setosus is concerned, although the optic 

 ganglion and optic nerve are present, the retina has undergone a com- 

 plete degeneration, and is now represented by a layer of undifferentiated 

 hypodermal cells. 



The eyes of Cambarus pellucidus present a somewhat different condi- 

 tion from that described in C. setosus. A longitudinal horizontal sec- 

 tion of the optic stalk of C. pellucidus is shown in Figure 2. The outer 

 surface of the stalk is covered with a cuticula (c£.) of uniform thickness, 

 and there is no indication of facets. Excepting at the apex of the 

 stalk, the hypodermis (lid.) is composed of a remarkably uniform layer 

 of cells. As in C. setosus, it is bounded on its deep face by a deli- 

 cate basement membrane (mb.). Both an optic ganglion (gn. opt.) and 

 nerve (n. opt.) are present, the latter being connected with the hypo- 

 dermis. In all these respects C. pellucidus resembles C. setosus, but 

 when the retinal part of the hypodermis in the two species is compared 

 a striking difference can be seen. The retinal hypodermis in C. se- 

 tosus (Fig. 1, r.) is, as we have seen, substantially like the remaining 

 hypodermis of the optic stalk. The retinal hypodermis in C. pelluci- 

 dus (Fig. 2, r.) is much thicker than the hypodermis of the stalk. With 

 this thickened region of the hypodermis the optic nerve is connected, 

 and there is no question, therefore, that this thickening represents the 

 rudimentary retina. Omitting minor details, the form of the thick- 

 ening is that of a plano-convex lens, the curved surface of which is 

 applied to the concave inner face of the cuticula at the distal end of the 

 stalk. The optic nerve is attached to the central part of the flat face 

 of the thickening. 



When the retinal thickening is carefully studied by means of radial 

 sections, one can see that it differs from the neighboring hypodermis 

 not only in thickness, but also in the fact that it contains two kinds of 

 substance : a protoplasmic material uniform with that of the rest of the 

 hypodermis, and a number of relatively large granular masses (Fig. 3, 

 con.). These granular masses contain two, three, four, or sometimes five 

 nuclei, and nuclei are also to be found scattered through the undiffer- 

 entiated protoplasmic substance. The nuclei in the granular masses 

 are slightly smaller than those in the surrounding portion of the hypo- 



