ENVIRONMENTAL CLASSIFICATION 33 



support their bodies, and thus makes it possible to dispense with or 

 to lighten special supporting structures. There are no other positive 

 characters common to all aquatic animals, for all types of structure 

 are represented in the water. Animals with gelatinous bodies per- 

 meated with water, Mycetozoa excepted, occur only in water, and 

 such forms are found among the most diverse groups: medusae, 

 ctenophores, worms (Alciopa) and their larvae, Heteropoda among 

 mollusks, salpae and eel-larvae {Leptocephalus) among the chordates. 

 Free feeding (i.e., non-parasitic) sessile animals are confined to the 

 aquatic habitat, for their food supply consists of suspended organisms 

 brought to them by currents, and they are thus able to dispense with 

 locomotion in search of food. 1 Ant-lion larvae, among terrestrial forms, 

 illustrate an approach to sessile feeding habits. Aside from the ar- 

 rangements for breathing the oxygen dissolved in water, there are no 

 limitations in the organization of aquatic animals. 



Marine animals. — Most groups of animals reach their maximum 

 size in the sea. The giant squids attain a body length of 6 meters with 

 arms of 11 meters; Tridacna, with a greatest diameter of 2 meters and 

 a weight of 200 kg., is the giant among bivalves; among arthropods 

 there is the Japanese crab {Kaempfferia kaempfferi) with a limb 

 spread up to 2 meters; the whale-shark, 10 meters or more in length, 

 represents the maximum for the fishes, while the sulphur-bottom 

 whale is the largest of living mammals. The unbroken connection of 

 the oceans, and the continuous diffusion of the sea water by means 

 of currents, tides, and storms, effect a general equality in the com- 

 position and amount of the substances dissolved in sea water. Con- 

 siderable variations occur only in limited areas. This widespread 

 uniformity of conditions is accompanied by an extremely wide dis- 

 tribution of many marine species of animals. 



Since the body fluids of marine invertebrates 2 ' 3 and of sharks are 

 approximately isotonic with the sea water in which they live, special 

 protective structures to prevent the osmotic exchange of substances, 

 by which the concentration of the body fluids might be altered, are 

 not required at the exposed surfaces such as the skin, gills, and mucous 

 membranes of the gut. Invertebrates in the Mediterranean accord- 

 ingly have a somewhat higher degree of salinity in their blood than 

 those of the Atlantic and North Sea. In the sharks and their allies 

 this osmotic balance between the blood and the surrounding medium 

 is maintained by the admixture of a considerable amount of urea 

 (2-3%) to the body fluids, which have a lower salt content than sea 

 water. In contrast, the body fluids of the bony fishes have a much 



