RHYTHMIC PULSATION 307 



view is supported by the low oxygen requirements of echino- 

 derms, to be considered subsequently, and the known presence" 

 of other modes of respiration than that associated with the respira- 

 tory trees. The phenomenon of increased amplitude of pulsa- 

 tion subsequent to forced inactivity in a pulsating structure is 

 also well known in the vertebrate heart, and may be seen, for 

 example, in such curves as those plotted by Vernon ('10) to show 

 the recovery of the heart-beat after perfusion with protoplasmic 

 poisons. 



The occurrence of anal rhythmic pulsations in holothurians is 

 not an isolated phenomenon, for such movements (with a sug- 

 gested similar function) occur among Enteropneusta (Willey, '99, 

 p. 244), both in the adult^ and in Tornaria (Willey, '99, p. 306), 

 and in decapod Crustacea (Miller, "10), etc. In the lobster and 

 crayfish the muscular arrangements for producing anal rhythm 

 are in a general way similar to those in holothurians, since in 

 the former case circular muscles and radiating muscles running 

 to the body wall also occur; furthermore, the coordinating 

 mechanism is a local one (Miller, '10, '12). 



c) Correlation with size. It has long been a matter of general 

 knowledge that the activity of animals varies with their size, 

 smaller animals being more active than larger ones. This rule 

 holds conspicuously for the execution of rhythmic movements by 

 animals of the same species. The rapidity of breathing move- 

 ments, etc., are not, however, simply proportional to the recipro- 

 cal of length or any other body measurement, but the empirical 

 curve expressing the relation between size, or weight, and activity 

 is almost invariably of a rather complex hyperbolic type. Poli- 

 manti ('13) has recently reviewed the literature of this subject, 

 and has supplied an additional example in the respiratory move- 

 ments of Octopus. The equation derived from his data is of 

 the form: 



y = a -\- bx + cx'^ -\- 



where y = weight, and x = respiration rate. 



8 I Iiave observed that the amputated posterior end (4-5 mm. long) of the 

 Bermudan Ptychodera sp. will pulsate in sea water. A sphincter ani is present. 



