PHYSIOLOGY OF SYNAPTULA HYDRIFORMIS 337 



The peristaltic waves, which assist locomotion through the sea- 

 weed, occur at irregular intervals, having no rhythmic sequence. 

 Each wave starts at the posterior end of the body as a marked 

 constriction some 2 to 3 mm. wide. The constriction passes an- 

 teriorly taking about 15 seconds to complete its course in a 

 Synaptula 45 mm. long. This rate, about 3 mm. per second, is 

 by no means constant, since in a given specimen waves were 

 found to take 18 seconds at one time and 16.6 at another. Waves 

 appear when a Synaptula is moving along a horizontal surface, 

 but not when it is crawling up a vertical wall. They also occur 

 when the animal is defecating. A fairly light mechanical stimu- 

 lus, such as a touch with a glass rod, causes a local constriction, 

 which often initiates a peristaltic wave if the stimulus is applied 

 to the middle region of the body, but not if applied to the extreme 

 anterior or posterior end. Similarly, local application of vari- 

 ous chemicals, such as KC1, NaCl, acetic acid, etc., calls forth 

 constrictions which may initiate a wave if applied to the middle 

 regions of the body. Peristalsis also occurs under the following 

 conditions: when the animals are fed carmine powder; when 

 they are returned to water at 27°C. (the normal) after having 

 been immersed for 3 minutes in sea- water at higher temperatures ; 

 when they are placed in calcium-free sea-water. 



Since peristalsis appears under such diverse conditions, it is 

 probable that this is a general response given to many kinds of 

 stimuli. At first it was imagined that only excessive stimula- 

 tion might provoke peristalsis; but this was found not to be the 

 case, since moderate stimuli, either mechanical or chemical, ap- 

 plied to the middle region of the body, nearly always produced 

 this response. It is probably the mechanical stimulation, due 

 to contact, which causes peristalsis when the animal is lying on 

 a horizontal surface or among sea-weeds. 



In two instances reversal of direction of the wave was noted. 

 In both cases several peristaltic waves passed completely from 

 the anterior to the posterior end immediately after cutting off 

 the head by a single stroke of the scissors. 



When moving through sea-weed the animals often curl the 

 last 5 mm: of the body about a branch and explore their sur- 



