Body Covering 19 



land-life can be elaborated before coming out on land. It is 

 not hard to see why the argument still boils. 



However, even the fish does not expose his living skin 

 entirely to contact with the world. He covers it with mucus, 

 or slime, secreted by the living cells and exuded onto the 

 surface. This is what makes a fish slippery. The quantity of 

 slime varies with the species. Some, like the pickerel, have a 

 large amount, as anglers can testify, while others, like the 

 carp, have comparatively little. 



The slime serves several purposes. In the first place, it 

 acts as a lubricant, permitting the body of the fish to slide 

 through the water without friction, thus increasing his speed 

 while reducing his energy expenditure. In the second place, 

 it protects him from attack by fungus, bacteria and other 

 parasites, forming a coating through which it is difficult for 

 these minute organisms to pass. In the third place, it helps 

 to make the fish water-tight, and this is highly important for 

 in most species the skin is in the nature of a semi-permeable 

 membrane. As you will remember from the days when you 

 studied physics, a semi-permeable membrane separating two 

 solutions of unequal strength allows liquid to pass through 

 it from the weaker solution toward the stronger. The fish's 

 internal fluids are somewhat salty: less so than the ocean, 

 but more so than fresh water. In the ocean, therefore, there 

 is a tendency for water to pass out of the weaker internal 

 solution into the stronger external solution j in fresh water 

 the reverse is the case. In other words, the fresh water fish 

 is in constant danger of becoming waterlogged, of absorbing 

 so much water that his internal machinery is no longer able 

 to function J and the ocean fish is in danger of losing water 

 until his insides dry up. The slime helps to save them from 

 these fates. 



It is because of the importance of the slime as a protection 

 against external parasites and against internal liquid un- 



