RHYTHMIC PULSATION 311 



to pulsation-frequency are of the same type as that found by 

 plotting Mayer's ('06, p. 8) data on diameter vs. pulsation-rate 

 in Cassiopea. 



In attempting to account for the relation of pulsation-rate to 

 size, it is possible to adopt the view that the rapidity with which 

 an animal executes a given act is a measure of the amount of 

 energy available for the performance of that type of work, and 

 that when operating under similar conditions the relative energy 

 content of different individuals in the same species may in this 

 way be compared. According to this interpretation, larger ani- 

 mals must contain less motor energy, proportionately, than do 

 smaller ones of the same kind. The observations of Tashiro and 

 Adams ('14), that the cardiac ganglia of large Limuli have a 

 lower output of CO2 per gram of nerve-substance than do the 

 (smaller) corresponding ganglia of smaller Limuli, and those of 

 Child ('13, p. 140), who found by the KCN method that young 

 (small) individuals of Planaria had a higher rate of metabolism 

 than larger ones, lend support to this general idea. 



III. COORDINATION OF THE PULSATING COMPLEX 



The pulsation of the cloaca exhibits a high degree of coordi- 

 nation in the action of a number of individual effectors. The 

 questions arise, From what center, if any, does the stimulus to 

 pulsation proceed, and by what means are the various muscles 

 brought to act in appropriate sequence? 



The following observations and experiments bear upon the 

 answers to these questions. The points to be considered are 

 (1) the cycle of pumping movements in the cloaca, and (2) 

 the cessation of these movements, with the exception of those 

 of the anal sphincter, during the expulsion of an expiratory 

 stream. It may be mentioned here that during defecation the 

 sphincter movements are not interrupted, and as a result the 

 faecal masses present beaded constrictions at regular intervals 

 along their length, the constrictions (fig. 11) being formed by 

 the pressure of the sphincter as it attempts to close. This applies 

 to Stichopus and Holothuria; but in Cucumaria there are no 



