148 PHOTIC SENSITIVITY OF CIONA INTESTINALIS 



cality which receives the stimulation. I shall describe the latter 

 first because of its comparative insignificance in the experiments to 

 be presented. 



The body of Ciona may be roughly divided into three parts : an an- 

 terior region occupied by the siphons and extending below the inter- 

 siphonal neural mass; a posterior region containing the viscera and 

 reproductive organs; and a middle region composed in the main, of 

 the translucent branchial sac. The posterior region is practically 

 insensitive to light. The anterior region, with the notable exception of 

 the area around the neural mass, is similarly insensitive. The middle 

 portion, however, is quite responsive to photic stimulatioii. 



Light focussed on different parts of this area resulted in a purely 

 local contraction of the circular and longitudinal muscle fibers. 

 When a part of this area on one side was exposed to light, the animal 

 curved on its long axis toward the exposed side. By repeatedly ex- 

 posing first one side and then the other to this local stimulation, the 

 animal could be made to curve back and forth at will. A similar 

 motion was secured by alternately exposing the dorsal and ventral 

 edges of an animal. As a result, it would swing back and forth, 

 dorsally and ventrally, subject to the frequency of stimulation. 



This sensitivity persists in its original strength after the intersi- 

 phonal ganglion has been removed, showing its independence of cen- 

 tral control. That we are not dealing here with a case of direct 

 stimulation of the muscular tissue (Parker, 1916) is shown by its be- 

 havior under chloretone anesthesia. When a Ciona is placed in a 

 solution of chloretone in sea water it loses this local photic sensitivity 

 simultaneously with its irritability to touch and to another type of 

 light stimulus which will presently be shown to be undoubtedly medi- 

 ated by a nervous mechanism. In all probability there is present 

 here a neuromuscular complex similar to the nerve net of lower in- 

 vertebrates, which is sensitive to light. Evidence for the existence 

 of such a nerve net has already been presented by Jordan (1908) 

 on the basis of his work on the mechanical stimulation of Ciona. 



2. Under ordinary conditions of experimentation, this sensitivity 

 and its local response are entirely masked by the otljer, more general, 

 reaction to light. When a Ciona as a whole is exposed to a source of 

 illumination, it reacts by a vigorous retraction of its siphons (Nagel, 



