RHYTHMIC PULSATION 317 



be observed that the cycle of movements constituting a com- 

 plete pulsation began at the cut end of the cloaca. 



TABLE 3 



Time relations, in seconds, of the phases of pulsation for the isolated cloaca in sea 



ivater 



Open phase. . 

 Closed phase. 



5.6-7.3 

 3.0-5.7 



7.0-8.0 

 5.2-5.6 



The direction of the current of water produced by the isolated 

 cloaca was studied with the aid of carmine suspended in sea 

 water, and also by small 'flags' of mucous or bits of thread at- 

 tached to the inner edge of the sphincter. The current was 

 directed anteriorly, as in the intact animal. The maximum 

 fluid pressure developed in the amputated pieces was ascertained 

 by inserting a glass tube of appropriate size into the anus, which 

 then contracted tightl}^ about the tube. The pressures were 

 never more than 1 cm. of sea water. In the intact animal the 

 cloacal pressure is much greater, since the body muscles then 

 play a greater part in the pumping. 



The pulsating sphincter exhibited a refractory period, such 

 that if the sphincter were touched at the very beginning of an 

 opening movement, it continued to open to its normal extent ; 

 but if touched at the edge when more than one-half open, the 

 brim closed down promptly, though some local retraction was 

 evident at the point stimulated (fig. 16). 



It was pointed out in a previous section that the rate of cloacal 

 pulsation was correlated with the size of the holothurian. It was 

 important to learn if this correlation persisted in the excised pos- 

 terior ends. The rate of pulsation in cloacal pieces of equivalent 



size (i.e., amputated at the level a a in fig. 1) derived from 



10 Stichopi of increasing lengths is tabulated in table 4, from 

 which it will be seen that, though some trace of this general 

 effect may be maintained, it is by no means clear cut; the rate 

 of pulsation in the isolated posterior ends was very rapidly 

 reduced to an approximately uniform level soon after excision. 



