THE CCELOMATE BODY 

 194 



of different strengths, water passes across them from the weaker 

 to the stronger solution (a phenomenon known as osmosis), 

 with consequent sweUing and increase of pressure. For most 

 animals living in the sea the semipermeability of livmg matter is 

 of little consequence, for their protoplasm has much the same 

 concentration of solutes as has sea water, so that there is little 

 movement of water. In fresh water things are different, and the 

 prevention of osmosis is important for animals living there ; it is 

 especially so in animals with a body cavity, for here not only 

 do the cell walls act as semipermeable membranes, but so do 

 the tissues themselves, causing water to be attracted into the 

 ccelomic fluid or blood. The most awkward situation of all is 

 that of estuaries, where there is a large daily change in the 

 external salt concentration. According to their osmotic relations 

 animals may be classified into two or three groups. First are most 

 of those which live in the sea, including the majority of inverte- 

 brates and the cartilaginous fishes ; their environment does not 

 normally change, and they have no means of coping with any 

 osmotic changes that do occur ; they are called stenohaline. If 

 put into fresh water they absorb it. Secondly, some invertebrates, 

 such as the shore crab and Nereis diversicolor, live where sea 

 water and fresh water mix, so that the salt concentration is 

 continually changing, and many bony fishes, such as the salmon, 

 pass from the sea into rivers and back again. They are able, to 

 a greater or lesser extent, to prevent the entry of water, or to 

 expel it after it has entered, and are called euryhaline, as the 

 salt concentration in their body fluids remains approximately 

 constant. Although living in a constant environment the marine 

 bony fish also have to regulate and are euryhaline ; their blood 

 has a lower salt concentration than sea water, so that water 

 tends to leave them. The animals which live permanently in 

 fresh water are also able to regulate their salt concentration in 

 the sense that they can cope with a tendency of water to enter, 

 but they may for various reasons be unable to survive in sea 

 water, so that while osmotically they are euryhaline ecologically 

 they make a third class. Osmotic regulation may be carried out 

 by a special organ, such as the contractile vacuole of Protozoa, 

 but more often use is made of an organ which has other functions 

 as well. The shore crab and fish use their gills, and most eury- 

 haline coelomates their excretory organ. In vertebrates the relative 

 impermeability of the skin is important. 



