28 ^^« Imroductlon to Zoology. 



temperature consequeutly varies with that of the luedium in which they 

 inove, are called " cold blooded " while the warm blooded are those who, 

 ou the contrary, preserve a teiuperatiue uniform, or nearly so, without 

 reference to the temperature around them. The reptiles and fishes, and 

 all below them, are of the former description ; the birds and the mammalia 

 are of the latter. 



Haviii"- considered the four principal solids, and the all-important fluid 

 of the animal frame, by means of which the great businesses of that frame 

 are carried on, we shall add a few words concerning their appearance in 

 different tribes of animals, and then pass at once to the businesses or 

 functions which they execute. 



The lowest animals are composed of gelatine ; they inhabit water, and 

 cannot be said to exhibit, though they may retain life out of it. 



A step higher, and the gelatinous cellular texture, of which the inferior 

 grade was composed, is not found firm enough for the increased wants of 

 the animal, and to this therefore is superadded albumen, a substance which 

 gives the requisite firmness, and which though perfectly flexible is tougher 

 and stronger than gelatine. Fibrous membranes are composed of a combi- 

 nation of albumen with gelatine, and they contain a proportion of albumen 

 increasing with the strength required. 



When gelatine almost entirely disappears, and the structure is formed of 

 albumen, combined with a small proportion of calcareous matter, it is called 

 cartilage, a dense substance, containing usually no fibres, and of a white 

 or bluish white hue. It forms the skeleton of the lower, or, as they are 

 called, the cartilaginous fishes, (shark, ray, sturgeon) of very young mam- 

 malia, and parts of that of older ones. The articulating extremities or 

 joints of the bones are tipped with it j lines of it connect the lower ribs 

 with each other and with the sternum, and it forms part of the windpipe 

 and organ of voice, together with the prolongation of the fauces and nostrils. 



Where cartilage is found from its flexibility to be insufficient, a harder 

 and more earthy material is employed. Where this earthy matter is car- 

 bonate of lime, as in the testaceous mollusca, it is exterior to the animal, 

 and is called shell. When it is in part carbonate and in part phosphate 

 of lime, with a predominance of the latter, being still, as in the Crustacea, 

 exterior to the animal, it is called crust. And where, as in the higher 

 fishes, reptiles, birds, and mammalia, it is almost entirely phosphate of 

 lime, and forming an internal skeleton, it is called bone. 



Having considered the material out of which the organs are wrought, we 

 shall next pass to their functions. 



All organic life, whether animal or vegetable, necessarily implies two 

 essential conditions ; these conditions are nutrition and reproduction : — 

 nutrition, by which it appropriates materials for the growth and develop- 

 ment of its parts while in a growing, and their sustentatiou in a mature 

 state ; and reproduction, which by the primary law was impressed npon the 



