METABOLISM 3II 



by their aquatic medium. The first and foremost of these arises from the 

 fact that Cetaceans hve in sah water, where thirst can become as oppressive 

 as in the most arid of deserts. 



An animal's body consists largely (about 70 per cent) of water with a 

 salt concentration of from about 0-93 per cent. If the animal is to stay 

 alive, this concentration must be maintained under all conditions. Aquatic 

 animals vv'ithout backbones, e.g. crustaceans, squids and water-snails 

 are more fortunate in that, unlike vertebrates, their body fluids have 

 roughly the same salinity as the sea in which they live. Aquatic vertebrates, 

 on the other hand, are quite definitely not so well adapted to their environ- 

 ment. While sharks and rays took to sea water fairly quickly owing to a 

 special adaptation of their blood to the saline environment (urea), bony 

 fishes {Teleostei) took 200,000,000 years to do so completely. For a number 

 of reasons not yet perfectly understood, the salinity of the body fluid of 

 vertebrates is very much lower than that of non-vertebrates, and hence 

 well below that of the sea-water. This is true even of marine fishes, which 

 have only a somewhat higher salt concentration than their fresh-water 

 relatives. In a number of places (e.g. the oral epithelium, the intestines, 

 and the gills) the body fluid is separated from the sea-water by only a thin 

 wall which acts as a semi-permeable membrane, i.e. a membrane which 

 will allow water, but not salts, to pass. Since there is a tendency to keep 

 the salt concentration equal on either side of the membrane, water will 

 constantly be withdrawn fiom a body with a lower salt concentration, 

 thus tending to expose the animal to dehydration in the midst of a sea 

 of water. 



Of course, this does not happen in fact, or else bony fish would not have 

 been able to live in the sea for 200,000,000 years. In the first place bony 

 fishes take in water in the form of food and sea-water, thus counteracting 

 the osmotic eflfects. The sea-water, however, contains more salt than 

 fish blood, and this surplus must be removed. Fish can rid their blood of 

 the extra salt because their gills are provided with special cells for this 

 purpose. 



Now, aquatic mammals lack any such special cells and must solve the 

 problem of their low salinity in a diflferent way. The salinity of their blood 

 and other body fluids, though somewhat higher than that of terrestrial 

 mammals, is still considerably lower than that of sea-water, so that whales 

 and dolphins do in fact lose water through the intestines, and other parts 

 of the body. This was shown experimentally by Fetcher in 1940 and in 

 1942. He pumped just under half a gallon of sea-water into the stomachs 

 of each of two Bottlenose Dolphins and found that after some time, the 

 faeces had a salt concentration equal to that of the blood, so that water 

 must have been withdrawn from the body by the intestines. 



