150 NEW ZEALAND NATURE-STUDY BOOK 
and far back—so that a cavity is formed by the water in 
which the gills are constantly moistened. 
The body of the fish always feels cold.—This is not 
because it lives in the water, for other animals which live in 
water are not cold-blooded e.g. the Whale and the Seal. Fish 
are sluggish breathers and take in so little air that it is not 
sufficient to warm the blood in their bodies. The circu- 
lation of the blood moreover is very slow—the heart being 
a relatively weak organ. In the case of a warm-blooded 
animal such as a Rabbit, the heart not only drives the blood 
to the lungs, but forces it over the body through special 
blood-vessels—the heart of the fish merely sends the blood 
to the gills, from which it finds its way slowly through the ~ 
blood-vessels of the body and back again to the heart. 
Locomotion. The fish swims by means of its tail and 
fins ; but the former is by far the most important organ of 
locomotion. The tail, which is controlled by strong muscles 
is moved rapidly from side to side; this brings about a back- 
ward and forward movement as well—a process which forces 
the animal through the water just as a man propels a boat 
by the action of a single scull passed over the stern. The 
tail-fin also acts as a rudder enabling the fish to alter its 
course with rapidity. 
The fish tapers from the back to the under surface, hence 
the heaviest portion is that along the back, so that by the 
laws of gravitation this part ought not to be uppermost. — Its 
position is maintained by the paired fins which act as 
balancers, as may be shown by fastening the pectoral and 
pelvic fins of a living fish to the sides of its body—ait this be 
done the animal will turn over on its back. Again a dead 
fish floats on its back, for the fins cannot balance it as they 
did when alive, and the heaviest part naturally takes the 
lowest place in the water. The paired fins moreover help 
to control the movements of the fish in other ways. The 
