46 ZOOLOGY. 



deed, to be only water charged with a certain amount of 

 organic particles; hut in animals higher in the scale of heing, 

 the humours cease to he of the same nature, and there is one, 

 distinct from all the others, destined to nourish the body : 

 this fluid is the blood. It not only nourishes the body, but 

 is the source whence are drawn all the secretions, such as the 

 saliva, urine, bile, and tears. 



79. In mammals, birds, reptiles, fishes, and in most 

 animals of the class annelides, the blood is red. But in the 

 greater number of the lower animals the blood presents 

 various hues and density, being often thin or watery, and 

 slightly yellow or green, rose-coloured or lilac. It is diffi- 

 cult, therefore, to be seen, and for a long time these animals 

 were called bloodless or exsanguineous. 



Those animals with white blood are very numerous ; 

 all insects, for example. 5 * The Crustacea of all sorts have 

 only white or pale-coloured blood; and in this category 

 may be placed all the mollusca, zoophytes, and intestinal 

 worms. 



80. By the use of the microscope we discover that the 

 blood of a red-blooded animal is composed of a yellowish 

 transparent liquid, called serum, and of a number of small 

 solid corpuscles which float in the serum, called blood glo- 

 bules. 



81. Globules of the Blood. Before birth, the globules 

 have dimensions and even a form different from what they 

 afterwards acquire. Thus in the chick the globules of the 

 blood are at first circular ; and it is only at a more advanced 

 period of incubation that the globules assume an elliptic 

 form. After birth the globules never vary. 



In all animals of the same species the globules have the 

 same form and nearly the same dimensions. It is not so 

 with different species. It has been observed also that in 

 animals of the same class the blood globules resemble each 

 other, but differ from those of other classes. In the first, 

 they have nearly all the same form; but in those of a diffe- 

 rent class, they may vary not only in dimensions but even in 

 form. 



Thus, in man (Fig. 36) and in most mammals the 

 globules of the blood are circular. In the camel and lama, 



* The red-coloured liquid which appears when the head of these insects is 

 crushed is not blood, but comes from the eyes. 



