102 COMPARATIVE ZOOLOGY. 



snded fluid, and retnni it to tiie blood-vessels. At the 

 same time, the waste products of the tissues are collected 

 and brought through the venous capillaries, veins, and 

 lymphatics to the excretory organs. Tlie special function 

 of the several constituents of the blood is not wholly 

 known. The colorless corpuscles in Vertebrates are sup- 

 posed to be the source of the red disks. The latter are 

 the carriers of oxygen, which is taken up by their red 

 matter (haemoglobin) in the lungs, and given up to the 

 tissues. The same office is performed by the bine color- 

 ing-matter (hsemocyanin) in tiie blood of certain Inverte- 

 brates, as the Squid and Lobster. The carbon-dioxide is 

 taken up mainly by the plasma. 



Like the solid tissues, the blood, which is in reality a 

 liquid tissue, is subject to waste and renewal, to growth 

 and decay. The loss is repaired from the products of 

 digestion, carried to the blood by the lacteals, or absorbed 

 directly by the capillaries of the digestive tract. The 

 white corpuscles are probabl}' prepared in many parts of 

 t])e body, especially the liver, spleen, and lymphatic glands. 

 In the lower organisms, the nutritive food is prepared by 

 contact with the tissues, without passing through special 

 ■organs. Lymph differs from blood chiefly in containing 

 less albumen and fibrin, and no red disks. Chyle is 

 lymph loaded with fat globules, and is found in the lac- 

 teals and vessels connected with them during the absorp- 

 tion of food containing fat. 



