114 LIFE IN THE SEA [CH. 



the source of soluble food-stuff is thus continually 

 being changed more of the latter can be taken up 

 than if the water were at rest. 



Thus we can account for the evolution of an 

 alimentary canal, mouth and anus, gills, blood cir- 

 culation, and respiratory movements and mechanism, 

 solely by reference to the necessity for the increased 

 absorption of dissolved food-stuff, and without 

 reference at all to the digestion of solid food 

 matter ; and it is the increase in size of organisms 

 which has led to the necessity for this develop- 

 ment. 



But nevertheless there is a digestive mechanism 

 in many animals, and we have also to account for the 

 development of this even if the absorption of soluble 

 food remains a factor in their nutritive process. Now 

 the key to this is the establishment of an internal 

 cavity. Some insectivorous plants have such an 

 internal cavity, evolved by leaf modifications. It 

 happens that small animals enter these cavities and 

 they are utilised as sources of food-stuffs by the 

 plants, but they are not necessarily digested. Utri- 

 cularia does not digest entrapped flies, but the 

 latter die in the bladders and undergo bacterial 

 decomposition. Drosera does possess a digestive 

 fluid but this only carries on the process as far as 

 the formation of peptones. Nepenthes forms a true 

 proteolytic enzyme. We can easily conceive of the 



