i86 THE BIOLOGY OF THE SEA-SHORE 



according to Putter, is simply that these animals are not 

 dependent for their food upon the ingestion of planktonic 

 organisms, since the quantity of these is much too small to 

 be able to satisfy the requirements of the population of larger 

 marine animals. To give a concrete example, Putter 

 estimated that the sponge Suherites domuncula, on the basis 

 of its carbon requirements, would need to capture every 

 hour the whole of the plankton contained in 242 litres of 

 sea- water, that is to say, in an amount of water equivalent 

 to about 4000 times its own bulk. But such a feat is mani- 

 festly impossible, since the amount of water actually passing 

 through the osculum of the sponge would not exceed 

 300 c.c. The question then arises as to the way in which 

 these animals do nourish themselves and as to what other 

 sources of food are at their disposal. According to Putter, 

 another source of food is present in the numerous organic 

 carbon and nitrogen compounds occurring in solution in 

 the sea-water, and the animals nourish themselves by absorb- 

 ing these through their whole surface. These substances 

 are formed as the result of plant-metabolism and are given 

 up to the sea-water, possibly after undergoing considerable 

 changes by the agency of the numerous bacteria adhering 

 to the algae. Organs such as the gills of molluscs, the 

 branchial sacs of Ascidians, the cirri of Polychsets, which are 

 of such frequent occurrence, would not merely be of use for 

 respiration, but would serve the purpose of absorbing 

 dissolved foodstuffs as well. Similarly, the primary function 

 of an alimentary canal, which is present in all but the very 

 lowest invertebrates, is not the one which we usually attribute 

 to it, but rather that of providing an increased amount of 

 surface through which the absorption of liquid material 

 may proceed. The process of taking in and digesting solid 

 food, to which we are so accustomed in higher animals, 

 has only been secondarily acquired by the gut. 



The method of feeding of a number of marine animals 

 would thus on Putter's view be essentially saprozoic, and 

 should his theories prove correct, our sharp distinction 

 between the feeding methods of marine plants and animals 



