814 PROFESSOR J. STEPHENSON ON 



are complete ; there is a well-developed dorsal vessel separate from the intestine, and the 

 contractions of the two are quite unrelated to each other. 



3, The Bearing of the Observations on Putter's Theory. 



The introduction of water at the anus, and its propulsion upwards along the intestine, 

 has throughout the present paper been referred to, in accordance with the usual custom, 

 as a respiratory process. The assumption is that the oxygen carried in solution diffuses 

 from the water through the intestinal epithelium into the fluid contained in the 

 intestinal sinus, or network of capillary vessels in the wall of the gut, while carbon 

 dioxide is given off" in exchange. 



It has, however, been hinted (p. 776) that other dissolved matters may be thus 

 introduced. There is obviously no reason why, if suitable substances in solution gain 

 an entry into a cavity whose walls have in a special degree, as have those of the intestine, 

 the power of absorption, those substances should not be absorbed, just as the nutritive 

 matters which enter by the mouth and are reduced to solution in the anterior part of 

 the tract are absorbed. Though intestinal respiration is a well-established term, it is 

 at least possible that the process may not be exclusively respiratory. 



In these circumstances the views of Putter become of interest. Putter holds that 

 in the case of aquatic animals a considerable part — even the greater part — of their food 

 comes, not from solid matters introduced into and reduced to solution in the alimentary 

 tract, but from the dissolved organic matter of the medium in which they live, which 

 is absorbed, it may be through the alimentary walls, or in other cases through some 

 portion of the surface of the body. 



The results of Putter's earlier investigations are conveniently presented by Johnstone 

 (25), from whose work I take the substance of the two following paragraphs : — 



Putter found, in the case of the sponge Suberites domuncula, that an animal of 

 about 60 grammes weight required per hour 0'92 mgm. of carbon. If the sponge is to 

 obtain its food by eating plankton organisms (Lohmann's estimates of the density of 

 plankton in the open Mediterranean being assumed true for the bay of Naples), it would 

 be necessary that it should capture, per hour, all the plankton contained in 242 litres 

 of sea-water, that is, about 4000 times its own volume. The volume of water passing 

 through tlie osculum in this time is, however, only about 300 c.c, and this quantity 

 contains only sTo^h part of the carbon required by the sponge. 



Accordmg to Putter's estimate, the carbon requirements of the animal are contained 

 in a dissolved form in so small a quantity of sea- water as 14 '2 c.c. Thus, even if we 

 assume that all the carbon compounds contained in sea-water are not capable of being 

 utilised as food, and that the absorption coefficient is not a very high one, it is still the 

 case that the water circulated through the cavities of Suberites may contain enough 

 carbon to supply the animal's requirements. Putter also suggests that the respiratory 

 surfaces of, for example. Molluscs and Ascidians are really surfaces for the absorption 



