TEMPERATURE OF COLD-BLOODED ANIMALS. 333 
animals whose vital actions are the most varied and energetic, 
and in which an interruption to any one of them most 
speedily brings the rest to a stand. The inquiry into the 
amount of heat generated by different animals, and into the 
sources of its production, is one, therefore, of great practical 
importance. 
404. Our knowledge of the heat evolved by the lower In- 
vertebrated animals is very limited; but it is probable that 
in most of them the temperature of the body follows that 
of the element they inhabit, keeping a little above it for a 
time, if it be much lowered. Thus, when water containing 
Animalcules is frozen, they are not at once destroyed; but 
each lives for a time in a small uncongealed space, where the 
fluid seems to be kept from freezing by the heat liberated 
from its body. The temperature of Earthworms, Leeches, 
Snails, and Slugs, ascertained by introducing a thermometer 
into the midst of a heap of them, is usually about a degree or 
two above that of the atmosphere; and they also have the 
power of resisting for a time the influence of a degree of cold, 
which would otherwise immediately freeze their bodies. 
405. In the cold-blooded Vertebrata, also, the heat of the 
body is almost entirely dependent upon that of the surround¬ 
ing element. Thus most Eishes are incapable of maintaining 
a temperature more than two or three degrees higher than 
that of the water in which they live; and the warmth of 
their bodies consequently rises and falls with that of the sea, 
river, or lake they may inhabit. There are, however, a few 
marine Fishes which have the power of maintaining a tem¬ 
perature 10 or 12 degrees higher than that of the sea; and 
these are peculiar for the activity of their circulation, and for 
the deep colour of their blood, which possesses red particles 
(§ 229) enough to give to the muscles a dark red colour, like 
that of meat. The Thunny , a fish which abounds in the 
Mediterranean, where there are extensive fisheries for it, is 
one of this group.—It is to be remembered that the animals 
of this class are less liable to suffer from seasonal alternations of 
temperature, than are those which inhabit the air. In climates 
subject to the greatest atmospheric changes, the heat of the 
sea is comparatively uniform throughout the year, and that of 
deep lakes and rivers is but little altered. Many have the 
power of migrating from, situations where they might other- 
