234 BUIvLETIN OF THE BUREAU OF FISHERIES. 



distal pigment cells. One shrimp was kept in darkness 38 days, but the change was 

 the same whether the interval was one of a few hours or weeks." The true significance 

 of this response was clearly established by Exner in his remarkable work on the phy- 

 siology of faceted eyes in insects and crabs, published in iSgi.* It was shown that the 

 distal and proximal pigment cells or the "iris" and "retina" pigment moved in opposite 

 directions in response to waning light, the former in its "night position" moving up to 

 the cornea and leaving the refractive cone exposed and the latter crowding down upon 

 the basement membrane, thus exposing the sensitive tip of the rhabdom. In the 

 "day position" the converse movement takes place when the eyelet is completely 

 isolated, and only those rays which are parallel to its long axis can enter and reach the 

 rhabdom.'= When the pigment screens are separated and drawn wide apart at night, on 

 the other hand, light rays of any angle can pass freely from one ommatidium to another 

 to be refracted by the exposed cones upon the upper ends of the exposed sensitive rods. 

 The response is thus an adjustment to economize light, though at the expense of clear- 

 ness of image. At dusk the lobster can presumably distinguish moving objects, but 

 only dimly, since the eye at this time can produce no clear mosaic images. 



The compound eye of the house fly is said to have about 4,000 facets, that of a 

 dragon fly 20,000, while in a 12-inch lobster I estimated the number to be 14,000. 

 Assuming that the ommatidia are equally well isolated and equally sensitive in each 

 case, the relative efficiency of mosaic vision in insect and crustacean would be proportional 

 to the number of facets. Upon this showing the lobster has a rather poor eye when we 

 consider the unfavorable medium in which its visual powers must be exercised. The 

 image produced by this organ, as ^^xn^T showed by a photograph made through the 

 medium of the faceted insect eye itself, is single and upright; sight is attended by 

 great loss of Ught, and must be very imperfect except for short distances and when the 

 animal is moving in shallow water strongly lighted. The fact that the lobster is most 

 active at night, that it is abundantly supplied with tactile organs for feeling its way 

 about, and that the greater part of its life is spent at depths where clear vision is 

 impossible for lack of light, show us further that its visual organs can play but a subor- 

 dinate part in the activities of its daily life. 



SENSORY HAIRS. 



Certainly the most numerous and probably the most important sense organs of 

 crustaceans generally are the sensory hairs or setse, which are all of epidermic origin. 

 Each hair consists of a hollow, conical, or nearly cylindrical shaft of chitin, continuous 

 with the general cuticular basis of the shell, and is associated with one or more sensory 

 nerve elements connected with the central nervous system. 



o Memoirs of the National Academy of Sciences, vol. v, 4tli mem., p. 454. Washington, 1893. 



6 Exner, Sigm. Die Physiologie der facettierten Augen von Krebsen und Insecten. Leipzig, Wien, 1891. 



cit has been found by Congdon that increased temperatures cause movements in the pigment cells, which are probably of 

 a non-adaptive character and are reverse in direction to those caused by light. See Congdon, E. D : The effect of temperature 

 on the migration of the retinal pigment in decapod crustaceans. Journal of Experimental Zoology, vol. iv, p. 539-548. 1897. 



