PHYSIOLOGY OF SYNAPTULA HYDRIFORMIS 345 



The same effect was observed with |M CaC! 2 , with MgS0 4 

 and with MgCl 2 . 



In calcium-free sea-water, the intestine writhed and twisted 

 so violently that it was impossible for nearly a minute to count 

 the pulsations. It then became quieter, and also rather opaque, 

 though it beat at its regular rate for at least 45 minutes. After 

 this it twitched irregularly, and finally after about one hour all 

 movement ceased. 



The only single component of sea-water, therefore, in which 

 pulsation continues at all is NaCl, and even here pulsation ceases 

 in a very few minutes after immersion in such a solution. NaCl 

 + KC1 + CaCl 2 forms a more 'balanced' solution than NaCl + 

 KC1 + MgCk + MgS0 4 , since regular pulsation continues in 

 the former mixture for nearly an hour, but ceases in the latter 

 in some 2 minutes. These results are in perfect accord with those 

 obtained in the study of the cloaca of Holothuria by Crozier 

 ('16 a). 



There are no histological studies of Synapta in which nerves 

 or nerve cells have been shown to be present in the intestine. Its 

 behavior, however, strongly suggests the presence of a nerve net. 



3. The effects of external stimulation 



Synaptula, like the majority of the lower animals, exhibits 

 but few types of reactions. To nearly every external stimulus 

 applied to the body-wall it responds by a local constriction at 

 the point stimulated, or if the stimulus is of sufficient strength, 

 by a shortening of the whole body. The local constrictions are 

 brought about by the contraction of the circular muscles of the 

 regions stimulated and are called forth by such stimuli as touch, 

 light, and chemical agents. If the stimulus is sufficiently vig- 

 orous, the constriction may be very deep and initiate a peris- 

 taltic wave which moves toward the anterior end of the body. 

 The shortening of the body is, of course, brought about by the 

 contraction of the longitudinal muscles. The antagonistic ac- 

 tion of these two sets of muscles and their components in differ- 

 ent regions of the body determines, of course temporarily, the 

 shape of the animal. For example, in the contracted state of all 



