CHAPTER VI. 



The J^lood in the Invertebrata. 



In auimals of the simplest structure all the fluids of 

 auimal economy resemble one another. " It seems, indeed, 

 to be only water charged with a certain amount of organic 

 particles ; but in animals higher in the scale of being, the 

 humours cease to be 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." In the Mam iik ilia, Aves, 

 Hrpfilia, Amphihia^ Pisces, and in most of the Annelida, the 

 blood is of a red colour. But in the greater number of the 

 Invcrtchrafa the blood presents various colours and densities, 

 being often thin or watery, and slightly yellow or green, 

 brown, rose-coloured, or lilac. The majority of the Inver- 

 tehrata have white blood ; e.g., the Inseda, Crudacea, Mol- 

 lusca, &c. 



The blood of the Invertehrata, like that of the Vertebrata, is 

 not homogeneous, for it consists of a transparent or semi-trans- 

 parent liquid, and a number of small, solid corpuscles, which 

 float in this liquid. 



In the higher animals the corpuscles are of two kinds, red 

 and colourless ; but in the Invertehrata there are, as a rule, 

 only colourless corpuscles. The red blood of Annelides is 

 different from the red blood of the Vertebrata, inasmuch as 

 the ]>lasma is coloured, and the corpuscles are colourless in 

 the former, while in the latter the plasma is colourless, 



