338 W. J. CROZIER 



The cloacae exposed to light did not recover on return to the shade- 

 Light exerted its typical effect upon the excised cloacal ends, and at 

 a rate which is comparable to that at which it causes the disappearance 

 of sensitivity to shading in the intact animal (Crozier '14, '15). 



e) Summary. Lack of oxygen would appear to be much less 

 powerful as a depressant of rhythmic activity than the pi-esence 

 of urea,i" CO2, and other typical metabolic products (the effects 

 of acids will be discussed subsequently in a separate section) , 

 though the suppression of oxidations by KCN is rapidly effective 

 in stopping movement. In those cases in which light produces 

 toxic modifications in the animal (H. surinamensis, H. captiva), 

 this agent also causes rapid cessation of rhythm. 



Vn. INFLUENCE OF OSMOTIC PRESSURE 



a) Dilutions of sea water. According to Henri et Lalou ('03), 

 the membranes of Stichopus regalis which are exposed to contact 

 with water are almost perfectly semi-permeable, the salt con- 

 centration in the ambulacraP^ and perivisceral fluids being nor- 

 mally slightly less than that in the sea water. They found that 

 S. regalis readily accommodated itself to moderate dilutions of 

 the sea water in which it was placed. 



In the experiments about to be described it was found that 

 diluted and concentrated sea water exerted characteristic effects 

 upon cloacal pulsation. It should be remembered that in these 

 experiments with the isolated cloacal ends the medium had 

 access to the internal surfaces of the body wall and cloaca, and 

 further that the cut surfaces of the integument and cloaca at the 

 exposed end were freely open to its action. 



The pulsation-rate of amputated cloacal ends was observed in 

 sea water at the following volume percentage concentrations: 

 0, 25, 50, 75, 90, 95, 100, 110, 120.1^ The history of typical indi- 



1- According to Henri et Lalou ('03) the membranes of Stichopus regalis are 

 impermeable to urea, but in my experiments a cut surface was exposed to the 

 action of the fluid. Furthermore, the remnant of the body cavity was freely 

 open. 



" From the Polian vesicle. 



^^ The dilutions from 100 per cent were made by the addition of rainwater; 

 the 110 per cent and 120 per cent solutions were made by evaporation of sea water 

 to the required volume. Normal Bermuda sea water contains about 36.8 p. p.m. 

 salt (Mark, '13). 



