RHYTHMIC PULSATION 



319 



The rate of rhythmic movement is, however, very much con- 

 ditioned by the length of the cloaca included in the cut off piece. 

 In table 5 is given a summary of the history of three posterior 

 ends of different lengths cut from animals of the same size, which 

 illustrates this point. 



TABLE 5 

 Dependence of 'pulsation rate and duration upon the length of the excised piece. 

 A = complete cloaca; B = about three-fourths of cloaca; C = one-half of cloaca; 

 each from a Stichopus 26 cm. long 



^ Irregular movements. 



This suggests that the stimulus to pulsation originates in the 

 anterior part of the wall of the cloaca with its associating radiat- 

 ing muscles, in the sense that the structure which of itself pul- 

 sates in sea water faster than any other part of the cloaca is 

 located there, and that its activity carries with it that of the 

 other contractile parts. A somewhat parallel case is found in 

 the vertebrate intestine, where the transmission of pulsation 

 depends upon the myenteric plexus (Cannon and Burket, '13); 

 Alvarez ('14) found that the rate of pulsation of strips from the 

 small intestine varied inversely with the distance from the 

 pylorus. The localization of the origin of pulsation in the cloaca 

 of Stichopus will be further considered later in this paper. 



Implied in the above description is the view that the fastest 

 beating member of a pulsating complex determines the rate of 

 movement of the whole. This idea was put forward by Loeb 

 ('00 ^' p. 29) in his explanation of the reversal of pulsation in 

 the tunicate heart; the neatest demonstration of the operation 

 of this principle is probably found in an experiment of Mayer 

 ('11, p. 7), who grafted together a large and a small Cassiopea 

 and found that the faster beating medusa determined the pulsa- 

 tion rate of the whole mass. 



