RHYTHMIC PULSATION 



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opened when the wave had run to this level, with the result that 

 the whole cloaca was now wide open; 4) the anus then closed, 

 and 5) the body in the region of the cloaca, which had previously- 

 increased in diameter, now began to contract in such a way as 

 to indicate that a wave of constriction was passing forward on 

 the cloaca. This does not mean that the anal sphincter is the 

 seat of origin of the pulsation wave, but is merely a description 



Fig. 3 Diagramatic longitudinal section through the cloacal end, showing the 

 relation of the sphincter during the pumping of water into the respiratory trees. 

 Arrow indicates the course of the water. 



Fig. 4 Showing the form of the anal brim during spouting. The arrow gives 

 the direction of the current. 



Fig. 5 View of the posterior end of Stichopus (X yV)- The action of the anal 

 dilators is shown by the irregular form of the edge of the brim. 



of the pulsation as seen from the outside of the animal. When 

 the anus was closed, and especially immediately after its sphincter 

 had contracted and the constriction of the cloaca and body wall 

 were increasing the pressure on' the fluid within the cloacal 

 chamber, the edge of the sphincter appeared to be acting after 

 the manner of heart-valves, as indicated in figure 3. This was 

 verified by the inspection, from the cut end, of amputated cloacae. 

 Yet, when the cycle of movements began again with the opening 



