102 
COMPARATIVE ZOOLOGY. 
suded fluid, and return it to the 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. The 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 blue color¬ 
ing-matter (haemocyanin) in the 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 probably prepared in many parts of 
the 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. 
