RECAPITULATION. 507 



and no black pigment is visible at their extremities. Tell- 

 kampf as quoted by A. S. Packard in the American 

 N'aturalist, 187 1, stated that the eyes were larger in the 

 young of C. pelhicidus than in the adult. Garman (1889) 

 says that the eyes are more prominent in young specimens 

 of C setosiis, and appear to lack but the pigment. 



The minute structure of the eyes in the adult of 

 C. setosus and pellucidus has been described in detail by 

 G. H, Parker (1890), but I have not been able to discover 

 that any investigation of the development or of the structure 

 of the eyes in the embryos or young has yet been made. 

 The optic stalks are not only pointed, but smaller than in 

 eyed crayfishes. In C. setosus the end of the optic stalk 

 is covered with an undifferentiated cuticle of uniform 

 thickness, and similar to that of the rest of the body. No 

 indication of the facets characteristic of functional eyes is 

 present. The so-called hypodermis, the cellular layer 

 beneath the cuticle, is also undifferentiated : the only feature 

 in which the hypodermis of the retinal region differs from 

 that of other regions is that it contains two or three irregular 

 rows of nuclei instead of a single one. There is, however, 

 below the hypodermis within the eye-stalk an optic 

 CTanorlion of considerable size, and from this grancrlion there 

 passes a short narrow optic nerve which extends to the 

 hypodermis. The complicated structure of the retina which 

 exists in functional crustacean eyes has therefore completely 

 degenerated, so that it has reverted to the condition of 

 undifferentiated hypodermis. Pigment is entirely absent. 

 The condition was observed in three specimens, two of 

 which were 6 cm, long, the third 4*2 cm. 



In Canibartis pelltLcidus the condition of the cuticle was 

 the same, and the optic ganglion and nerve were present as 

 in setosus, but the structure of the retinal region was 

 different. Degeneration had not proceeded so far. The 

 rudimentary retina connected with the optic nerve is 

 thickened, forming a lens-shaped mass, which contains,, 

 besides nucleated cells like the ordinary hypodermis cells, 

 large multinucleated granular bodies. Parker gives his 

 reasons for considering these granular bodies as the de- 



34 



