180 



INVERTEBRATE PHYSIOLOGY 



of molting but of most of the other known endocrine processes of crusta- 

 ceans. The elaboration of neurosecretory droplets, and their transmission 

 through the axoplasm of the fibers constituting the sinus-gland tract, prob- 

 ably could be studied histologically, but the stimulus and mechanism of 

 release of the active material from the sinus gland may be more difficult to 

 explain. Are the fibers of the sinus-gland tract capable of transmitting 

 nervous impulses, and thus of participating in the release mechanism, or is 

 release effected by a mechanism as yet unknown? 



Retinal Pigment 



The compound eyes of the higher crustaceans, which constitute the prin- 

 cipal photoreceptors of these animals, are composed of a large number of 

 ommatidial units. Each ommatidium is usually equipped with three sets 

 of retinal pigments, which have a somewhat varied terminology in the 



—DP 



"RP 



^S-PP- 



Fig. 1. Ommatidia from the retinas of Palaemonetes vulgaris, 

 showing the structure and the positions of the retinal pigments 

 in L, light-adapted eyes ; D, dark-adapted eyes ; E, eyes from 

 dark-adapted animals injected with eyestalk extract; C, cornea; 

 DP, distal pigment; PP, proximal pigment; RP, reflecting pig- 

 ment; BM, basement membrane; RH, rhabdome. 



