RESPIRATION IN WATER 23 



The large Coelenterata with less than 1% organic dry sub- 

 stance have an extremely low metabolism, and water circu- 

 lates slowly by ciliary movements through the gastro-vascular 

 canals. The oxygen tension in this water is only slightly 

 lower than in the surrounding sea (Backman, 1915). Henze 

 (1910) found for Pelagia, the body of which is approximately 

 a half sphere with about 6 cm diameter, that an increase above 

 normal of the oxygen concentration of the water did not 

 increase the metabolism, an observation which confirms the 

 existence of a positive oxygen tension everywhere in the 

 tissue. There is reason to believe that even in much larger 

 Coelenterata {Cyanea may reach a diameter of 2 m) diffusion will 

 be sufficient. 



In a number of Actinia, Henze's experiments show a definite 

 dependence of the metabolism on the oxygen tension avail- 

 able; the consumption, for instance, in Actinia equina (3 speci- 

 mens in 500 ml water) rising from about 0.5 ml 2 /hour at a 

 tension of 55 mm to 1.4 at a tension of 220 mm. This points 

 to the permanent absence of oxygen in certain parts of the 

 tissue. When the O2 content sinks below 2 ml/liter, Pieron 

 (1908) has observed A. equina to move to the surface of the 

 water and expose more or less of its body to the air. When 

 access to the surface is prevented the animals will secrete some 

 mucus, close up completely, and enter upon a period of latent 

 life. 



The case of the Cirripedia will be mentioned below (p. 32). 



Respiration combined with circulation. Assuming a spherical 

 organism possessing a cuticle through which oxygen must 

 diffuse while the transport inside the cuticle takes place by 

 effective convection, we can calculate the tension difference 

 necessary for the diffusion of the oxygen used in metabolism 

 and we have 



AtT 

 C °~ 3D 



in which C , A, r are defined as above, while T is the thickness 

 of the cuticle and D the diffusion coefficient for 2 in the sub- 



