304 W. J. CROZIER 



of the sphincter, its edge was pulled back sharply, a process which 

 was also evident at the termination of spouting (fig. 4) and subse- 

 quently; this is the function of the minute muscles above de- 

 scribed, which are placed on the coelomic face of the anal brim 

 perpendicularly to the circular constrictors of the sphincter. 

 The opening of the sphincter therefore involves not only the 

 relaxation of the circular muscles but also the contraction of a 

 set of antagonistic dilators, whereas in closing the action of these 

 sets of muscles is reversed. During the opening of the anus the 

 edge of the sphincter showed very clearly the action of the dilator 

 muscle strands (fig. 5) . The closure of the brim was usually not 

 quick enough to catch all the water within the cloaca at the end 

 of an expansion period, because the general contraction move- 

 ment slightly preceded the constriction of the sphincter, so that 

 some of the contained water escaped before the brim was com- 

 pletely closed. The time relations of the phases of a single 

 pumping movement are indicated by the measurements sum- 

 marized in table 1. 



TABLE 1 



Time relations of the phases of anal pulsation, in seconds 



(1) Sphincter open. . 



(2) Sphincter closed. 



(3) Expiration 



LENGTH OF 8TICH0PUS 



In all the measurements of pulsation rate given in this paper, 

 a complete 'pulsation' begins with the opening of the anal 

 sphincter and ends with the start of the next succeeding opening 

 movement. In practice this interval could be timed with ex- 

 treme accuracy, since, immediately the anal sphincter began to 

 open, the whitish interior of the cloaca became visible in the 

 center of the dark brown or black field presented by the general 

 surface of the animal. The slowness of the movements also 

 greatly aided precision in their measurement. Five to ten pulsa- 

 tions were timed with a stop-watch, and the result reduced to 



