156 BULLETIN OF THE 



continuous layer of cells (Fig. \,hd.) with its distal face applied to the 

 cuticula and its proximal face bounded by a fine but distinct basement 

 membrane (mb.). The layer is throughout very nearly uniform in thick- 

 ness ; at least it is not thicker in the region of the retina than at many 

 other places, and the slight variations in its thickness are not in signifi- 

 cant regions. The only feature of the retinal hypodermis which would 

 suggest that it was unlike the rest is the somewhat closer crowding of 

 its cells. This manifests itself in the arrangement of the nuclei in two 

 or three irregular rows, instead of a single one. In other respects the 

 nuclei of the retinal region and the surrounding hypodermis are essen- 

 tially similar. 



The optic nerve (Fig. 1, n. opt.) consists of a poorly defined bundle 

 of nerve-fibres which extend from the optic ganglion to the hypodermis. 

 The nerve-fibres are doubtless intimately connected with the cells in the 

 hypodermis, for the basement membrane is interrupted where the nerve 

 and hypodermis are in contact. It is probable that the basement mem- 

 brane is reflected from the hypodermis to the optic nerve, although I 

 have not been able to observe this with clearness. 



Recent investigations support the conclusion that the retina in the 

 Crustacea is derived from the hypodermis. In C. setosus that portion 

 of the hypodermis from which the retina would be derived is scarcely 

 distinguishable from other parts of the same layer. The retina in this 

 species, therefore, has so completely degenerated that it has at last 

 returned to the condition of almost undifferentiated hypodermis. 



That the optic nerve still retains its connection with the retinal area 

 is, on the whole, not so significant a condition as one might at first sup- 

 pose. It is probable that the optic nerve arises in this species as it 

 does in the lobster. I have elsewhere (Parker, '90, p. 43) attempted to 

 show that in the lobster it is not an outgrowth from either the optic 

 ganglion or the retina, but that, as the ganglion was differentiated from 

 the hypodermis, the optic nerve remained as a primitive connection be- 

 tween these two structures. So long, then, as an optic ganglion should 

 be differentiated one might expect an accompanying optic nerve ; but 

 the nerve would be present as a passive connection between hypodermis 

 and ganglion, rather than as a structure which had retained that posi- 

 tion by virtue of its continued functional importance. 



The foregoing account of the eye in C. setosus is based upon obser- 

 vations on three individuals of this species: Two of these measured, 

 from the tip of the rostrum to the end of the telson, 6 cm. ; the third, 

 4.2 cm. In the three individuals the eyes presented essentially the 



