ON THE CONTROL OF THE RESPONSE TO SHADING IN 

 THE BRANCHI^ OF CHROMODORIS. 



By W. J. CROZIER. 



(Contributions from the Bertmida Biological Station for Research, No. 106, and 

 from the Physiological Laboratory, College of Medicine, University of Illinois.) 



(Received for publication, April 28, 1919.) 



The gill-plumes (branchiae) of the nudibranch Chromodoris zebra 

 are normally retracted within the branchial collar after a shadow is 

 cast on the animal (see Fig. 1). If the decreased light intensity per- 

 sists, the gill-plumes become reextended, but usually not so fully as 

 in the first instance. Characteristically, in the dark the branchiae 

 are concealed within their collared pocket, and in the light become ex- 

 tended to a degree depending on the light intensity. It has been 

 shown (Crozier and Arey, 1919) that the photic control of gill-plume 

 extension depends upon the activation of photoreceptors located 

 within, and in the immediate vicinity of, the plumes themselves, and 

 not upon the eyes of the nudibranch. The mechanism of this con- 

 trol is complex: (1) the integument in the region of the branchial 

 collar is photosensitive and when it is illuminated induces the pro- 

 trusion of the branchiae; (2) the branchiae themselves, through the 

 action of a self-contained nerve net, respond by contraction when the 

 intensity of the incident light is quickly decreased, but do not react 

 to an increase in light intensity; (3) if a sufficient number of the 

 gill-plumes is excited by shading, their individual contractions re- 

 flexly determine a retraction of the whole gill-crown, together with 

 the sphincterwise closure of the branchial collar. These responses 

 have nothing to do, directly, with the positive photo tropism of the 

 nudibranch, although, as Arey and I (1919) have intimated, it is 

 significant that the locomotive activities initiated by light in a 

 previously darkened and quiescent Chromodoris are correlated with 

 the protrusion of the chief respiratory organs. 



585 



