RHYTHMIC PULSATION 299 



derived from the study of the medusae, are applicable to these 

 movements in an echinoderm, in which the neuro-muscular condi- 

 tions are in some respects similar. The account here given is 

 essentially of a preliminary kind. This work has, I believe, re- 

 sulted in the discovery of a type of material peculiarly favorable 

 to quantitative studies, especially upon the nature of ionic 

 actions. 



The animal principally employed for this study was Stichopus 

 moebii Semper,^ both because of its availability in unlimited 

 quantity, and since its large size greatly facilitated operative 

 experiments. Unless indication is made to the contrary, it is 

 to be understood that in the subsequent pages this species is 

 always referred to. 



To Prof. E. L. Mark I am again indebted for the privileges of 

 the Bermuda Biological Station and the abundant fauna to 

 which it gives access. The work was carried out during the 

 summer of 1914. I wish to record, in addition, my obligation 

 to Mr. L. B. Arey for much incidental assistance. Prof. G. H. 

 Parker has read this manuscript, and has materially aided me 

 both by inspiration and by advice, for which my best thanks are 

 due. 



II. PULSATION IN THE INTACT ANIMAL 



a) Morphology of the pulsating parts. The parts concerned in 

 pulsation will first be described, and, following this, their normal 

 operations, as a basis upon which to interpret the experiments. 



The general relations of the contractile parts in the aboral 

 end of Stichopus will be evident from figures 1 and 2. As is 

 necessary in any pulsating system which does not include an 

 elastic supporting structure against which the muscles can work, 

 there are essentially two sets of opposed muscles: 1) the radiat- 

 ing muscles, producing cloacal expansion, and 2) the circular 



" I agree with Clark ('01) that the black, spotted, and uniform reddish-brown 

 forms of this Stichopus are specifically identical. They are physiologically 

 indistinguishable, except in the matter of pigment formation, and in this all 

 intergrades were found among the approximately 2000 individuals examined. 



