FISHES 275 



Hence the blood flows slowly through the body. This may be shown by 

 making an incision behind the gills into the branchial artery of a fish, 

 when the blood will be found merely to flow from the wound, whereas if an 

 artery is opened in any other vertebrate animal the blood is spurted forth 

 with great energy. Even with a more active circulation, fish could not be 

 warm-blooded animals, since the water which constantly bathes the gills 

 withdraws so much heat from the blood that a large quantity of heat 

 would be required to raise the temperature of the blood above that of the 

 surrounding water. (Compare warm-blooded aquatic animals and their 

 special provisions for the conservation of heat.) This, however, is 

 impossible, for fishes can only draw upon a small supply of oxygen 

 in the water, 1 cubic metre of water containing only about 20 grammes 

 of oxygen, without which life is impossible, as against 300 grammes 

 contained in a cubic metre of air. In proportion, however, as the 

 circulation is slackened, and the quantity of oxygen taken up by the 

 blood diminished, the assimilation of food materials, and the bodily heat 

 which is thereby called forth (see Part I., pp. 7 and 8), as well as the 

 total mental activity of the animal, are correspondingly reduced. 



Like all animals whose body temperature is subject to variation (see 

 p. 229), fishes are most active during the warm season of the year. On 

 a summer's day they may be seen playfully disporting themselves in the 

 warm water of our rivers and lakes. In the winter they retire to the 

 deeper and warmer strata of the water, and there pass through a kind of 

 hibernation ,■ some even bury themselves in the mud. 



6. Body Coverings. — Cold-blooded creatures like fishes do not require 

 any heat-retaining covering. (Why ? See p. 229.) Their body is generally 

 covered with scales ; in some rare cases large horny plates are developed 

 in the integument, whilst in others the skin is perfectly naked. (Give 

 examples. See also the scales of sharks.) When these scales overlap 

 in the manner of tiles, they are invariably directed backwards, so as not 

 to impede the animal's progress. The mucus secreted by the skin and 

 covering the surface of the body has a similar import, but also serves as 

 a means of protection against enemies. (Why? Compare, however, 

 the teeth and beaks of animals which prey on fish.) 



Like many reptiles and amphibians, which are capable of adapting 

 their colour to their surroundings, many fishes also are endowed with 

 the faculty of assimilating the colour of their skin to that of the ground 

 over which they swim. Even under a small magnifying power we may 

 distinguish in the scales of a perch or carp the pigment cells (see p. 253) 

 by which these changes are effected. 



7. Organs of Sense. — The eye is constructed on essentially the same 

 plan as in mammals (see Part I., p. 12). In the water, even at the 



