674 EVOLUTION OF ANIMALS Part V 



rous fishes have a relatively small absorptive surface and those of herbivorous 

 ones a very large one. In many bony fishes, the blind pouches or caeca which 

 open off the intestine just behind the stomach increase the digestive and ab- 

 sorptive surface. Such caeca as these are found in no vertebrates above the 

 fishes. A catfish has none, a sunfish has seven, and the king salmon of the 

 Pacific has over 200. Fishes have an extensive liver, in some so stocked with 

 oil that it helps them float. The pancreas is in small pieces, not easy to identify; 

 they contain the islets of cells that secrete insulin. Fishes are peculiar in having 

 the anal opening anterior, instead of posterior, to the urino-genital ones as it is 

 in other vertebrates. 



Breathing. Fishes breathe through their mouths and by means of gills. The 

 breathing mechanisms are shown in Figure 33.12 and are described further in 

 Chapter 13. Two arrangements prevent undue confusion of food and water. 

 The esophagus is tightly closed by circular muscles except when food is swal- 

 lowed. The arches supporting the gills bear inward-projecting rakers that keep 

 food from lodging on the gills. It is not always remembered that the oxygen 

 mainly available to aquatic animals is originally absorbed from the at- 

 mosphere. Oxygen in the composition of water, HoO, is not available. 



The amount of water in the goldfish bowl is of little help if its surface ex- 

 posure is too small. Trout keeping close to the brook bottom on a warm day 

 remind one that cold water sinks and that it holds more oxygen than warm 

 water. 



Water Content and Excretion. The gills are the main breathing organs, 

 but they are also excretory organs that control the salt and the water content 

 of the body and eliminate waste products. An important difference between 

 fresh- and salt-water fishes is in their water income and outgo. 



Fresh-water fishes continually absorb water mainly through the gills. It 

 passes through the semipermeable membranes and into the salty body fluids 

 according to the law of osmosis (Chap. 2). Fresh-water fishes must have the 

 income of water controlled or their bodies swell. Much of the nitrogenous 

 waste diffuses out across the gills. 



Salt-water fishes drink water and their stomachs are often found full of it. 

 Their gills excrete salt. Their kidneys eliminate ammonia and urea, but very 

 little water. They must conserve water because their body fluid is less salt than 

 the ocean water and in very small amounts water leaches through the semi- 

 permeable membranes wherever it can. Salt-water fishes must have their 

 outgo of water controlled or their bodies shrivel. 



Shad, salmon, eels, and others can adjust from salt water to fresh and vice 

 versa, but not suddenly. Young salmon cannot be dumped into salt water any 

 time; only when they have silvery guanin crystals (nitrogenous excretory 

 products) in the skin is the change safe. 



The Air Bladder and the Sounds of Fishes. The majority of fishes 



