iS SCIENCE PROGRESS 



the sea, and carbon dioxide given off. Thus we shall be com- 

 pelled to regard the ctenidia of some mollusca, the respiratory 

 trees of holothurians, the branchial sacs of ascidians, the 

 respiratory plumes of some worms, and other structures, such 

 as the appendages of cirripede Crustacea, as organs which have 

 the function of taking up dissolved food substances from the 

 surrounding sea-water. The acceptance of such a view will 

 probably clear up some difficulties which have presented them- 

 selves to the zoologist as the result of a too complete reliance 

 on the conclusions of comparative anatomy. Why, for instance, 

 should some sluggish and inactive animals possess respiratory 

 organs with an extraordinarily large surface ; or why should 

 many invertebrates possess a richly branched gut ; or why again 

 should a typical alimentary canal exist in some animals while it 

 is usually very difficult to demonstrate the presence of solid food 

 particles in the lumen of the latter? The cod, an active animal, 

 has gills which have not nearly such a large respiratory surface, 

 relatively to the mass of the animal, as the ctenidia of the 

 sedentary oyster. Pycnogonids possess a richly branched gut, 

 but the alimentary canal of most fishes is relatively much simpler 

 and has an internal surface much less in proportion to the mass 

 of the animal. It is very difficult to find sufficient food particles 

 in the gut of some lamellibranch mollusca to satisfy oneself as to 

 the materials on which these animals subsist. So also with the 

 pycnogonids, and with other marine invertebrata. 



If we assume, however, that a proportion of the food of these 

 creatures is to be derived, not from other organisms taken into 

 the gut and digested, but from carbon (and nitrogen) compounds 

 absorbed by the integument of the gills, or other " respiratory 

 organs," or by the general surface of the body, or by the 

 epithelium of the gut, such difficulties disappear. We need not 

 suppose that the alimentary canal is purely an organ for the 

 absorption of dissolved nutriment. It may be that this was its 

 primary function and that it was evolved merely as an extension 

 of the general absorptive surface possessed by the animals in 

 which it first appears. The ingestion of solid food may co-exist 

 with the absorption of dissolved food, just as in insectivorous 

 plants a part of the nutritive substances required by the organism 

 may be obtained by the digestion of the bodies of insects, while 

 the major part of the food of the plant is obtained by a photo- 

 synthetical process. In the carnivorous marine animals what 



