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 iTnTnp.rHa.te1y 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 FISHES 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- 



