vol.i.] Torrey. — Hydroida of the Pacific Coast. 41 



These experiments seem to demonstrate what was suggested 

 by the slowness of the orientation of the normal individual and 

 the method of lengthening the stem by increasing the turgescence 

 of the skeletal cells (since the diameter may increase at the same 

 time) viz., that the skeletal cells alone are susceptible to geotactic 

 stimulation, the muscles producing only such comparatively 

 rapid movements as the contraction of the tentacles and the 

 proboscis, and the bending of the column away from the perpen- 

 dicular against the stimulation of gravity. 



Response io tactual stimulation. A vigorous stimulus, such 

 as a pinch by forceps, results in a contraction of the stem within 

 two seconds, whether the stimulus is applied to a tentacle <>r the 

 proboscis or the proximal or distal portion of the stem itself. 

 Only that part of the stem contracts that is .uot invested in 

 perisarc, though the perisarc seems to be too thin to be an effec- 

 tive hindrance to contraction in the basal region. The fact may 

 be formulated in this way: A vigorous stimulus applied in 

 any region of the body produces a definite localized response. 

 The phenomenon reminds one forcibly of the behavior of Mimosa 

 under stimulation, and is due to the same immediate cause — a 

 change in the turgescence of certain large vacuolated cells, which 

 in Corymorpha form the axis of the stem. 



A stimulus too slight to produce any reaction when applied 

 to the stem, may be effective with a tentacle. The tentacle may 

 respond at once and independently of all the others, by shorten- 

 ing slightly, and waving toward the proboscis. This reaction is 

 the same, whether the stimulus be applied to the tip or base, 

 upper or lower side. If the stimulus be increased, all the tenta- 

 cles may contract, without any evidence of response in the stem. 



These reflex movements indicate the presence of a co-ordi- 

 nating mechanism which appears to have adaptive value for the 

 prehension of food and for protection. 



Regeneration. C. palma regenerates certain lost parts with 

 great readiness. Pieces of the stem produce remarkable cases of 

 hetermorphosis, which will be considered in another paper. A 

 few of the facts connected with the regeneration of the hydranth 

 may be mentioned here. 



Proximal and distal tentacles appeared on several regenerating 



