KHYTHMIC PULSATION 353 



In the latter case the edge of the sphincter continues to pulsate. 

 Pulsation continues during defecation. The isolated cloacal ends 

 do not 'spout;' they contract in response to mechanical, chemical, 

 and photic (shading) stimuh. The stimulus to spouting in the 

 intact holothurian is probably derived from a condition of ten- 

 sion in the body wall. 



The radiating muscles and connective-tissue strands of the 

 cloaca are necessary for the performance of pulsation. They 

 appear to act in connection with an integumentary nerve net. 



The results of experiments upon the cloacal termination of 

 Stichopus are in essential agreement with the data derived from 

 many previous studies of pulsating structures, such as those of 

 medusae, ctenophores, the arthropod heart, and the vertebrate 

 heart and intestine. The rhythm has a temperature coefficient 

 of the order of magnitude of that for chemical processes. The 

 rate of pulsation is related to the size of the animal (in Stichopus, 

 Holothuria surinamensis, H. captiva, and Cucumaria punctata) 

 in such a way as to suggest that the larger animals, which 

 pulsate more slowly, possess relatively less energy than do smaller 

 ones of the same species. Pulsation of the amputated aboral 

 end is readily depressed by urea, carbon dioxide, acids, and KCN ; 

 it is resistant to lack of dissolved oxygen. Either dilution or 

 concentration of sea water curtails the duration of pulsation. 

 Rhythmic movements do not continue for more than a few min- 

 utes in non-electrolyte solutions. The relation of pulsation to 

 the salts of sea water is essentially Uke that in other well known 

 pulsating systems; NaCl + CaJ^2 + KCl (in the proportions 

 found in sea water) enables pulsations and irritability to con- 

 tinue longer than with NaCl or with NaCl + CaCl2; but mag- 

 nesium, and particularly MgS04 (at least in neutral salt mixtures), 

 must be present to insure normal diastole. Calcium is intimately 

 concerned in contraction. In the series 



NaCl > LiCl > NH4CI > KCl (5/8 M) 

 the preservation of pulsation and irritabihty is successively less. 

 The pulsating mechanism is extremely susceptible to increase 

 in the hydrogen-ion concentration. The addition of NH4OH, 

 or other alkahes, to normal sea water assists in the preservation 

 of pulsation and irritability. 



