RHYTHMIC PULSATION 301 



piuscles of the cloaca, sphincter, and body wall, producing con- 

 tration of the cloacal region.'* 



The conditions are, however, much more complex than this 

 simple statement indicates, and it is necessary to consider the 

 distribution of the muscles more in detail. The third limb of 

 the intestine opens into the cloacal chamber {in.-clc, fig. 1) on 

 its ventral side. This opening is guarded by a sphincter, which 

 is kept closed except during defecation. At its anterior end the 

 cloaca gives rise to the stalk of the respiratory trees, also guarded 



Fig. 1 The posterior end of Stichopus moebii opened along the mid-dorsal 

 inter-radius; the body wall is somewhat distorted, so that only four of the five 

 longitudinal muscles are seen. From a living preparation (X 2|). 



a7i., anus. ms.'ent., mesentery. 



arb.brn., trunk of respiratory tree. nm.crc, circular muscles of the body 



nrb.brn'., small derivatives of the respi- wall. 



ratory trees, originating rather far mu.lg., longitudinal muscles of the 



down the stalk. When full of water body wall. 



these small branches pass backward rmi.r., radiating muscles of the cloaca, 



into the space about the posterior Some of them are shown cut. Only 



end of the cloaca. a few of the many strands present 



ar6.6rn.da:., right respiratory tree stem. are drawn. Some of the strands are 



arb.brn. s., left respiratory tree stem. of connective tissvie, and are not 



clc, outline of cloaca. contractile. 



in., intestine. a a, level of amputation in experi- 



in.-clc, union of intestine with the ments with the isolated cloaca, show- 

 cloaca, ing the range of this level. 



marg.an., anal brim. 



Fig. 2 A. The body wall of Stichopus moebii divided along the dorsal inter- 

 radius, and the cloaca removed (X ^). B. A portion of the anal brim flattened 

 out (X If). 



dil.an., dilators of the anus. mu.lg., longitudinal muscles. 



marg.an., anal brim. spht., circular fibers of the sphincter. 



mu.crc, circular muscles. 



'' Jordan ('13, '14) has dealt with the contractile properties of the so called 

 'connective tissue' of the skin of holothurians, and states his belief that this 

 tissue by its degree of slow tonic contraction acts in conjunction with the loco- 

 motor muscles. The pulsation movements which we are considering are, 

 as compared with the progression movements, very rapidly executed, so that 

 (aside from their role in maintaining a tense support for the radiating muscles) 

 these contractile elements of the integument are not discussed in detail. The 

 relation of salts to their degree of tonus will be studied in a subsequent paper. 



