xiii PHYLUM CHORDATA 91 



the form of flat oval discs (A.), the centre bulged out by a large 

 nucleus (nu.), but in mammals (B.) they are bi-concave, non- 

 nucleated and usually circular. They do not perform amoeboid 

 movements. 



The colour of the blood varies with the amount of oxygen taken 

 up by the haemoglobin. When thoroughly aerated it is of a bright 

 scarlet colour, but assumes a bluish-purple hue after giving up 

 its oxygen to the tissues. Owing to the fact that oxygenated 

 blood is usually found in arteries, it is often spoken of as arterial 



FIG. 731. Surface and edge views of red-blood corpuscles of Frog (A) and Man (B). nu. nucleus. 



(From Parker's Biology.) 



blood, while the non-oxygenated, purple blood, being usually found 

 in veins, is called venous. But it must not be forgotten that an 

 artery, e.g., the ventral aorta or the pulmonary artery, may contain 

 venous blood, and a vein, e.g., the pulmonary : vein, arterial blood. 

 The distinction between the two classes of vessels does not depend 

 upon their contents, but upon their relations to the heart and the 

 capillaries. 



In addition to the blood-vessels the circulatory system of 

 Craniata contains lymph-vessels or lymphatics (Fig. 728, ly.). In 

 most of the tissues there is a network of lymph-capillaries, inter- 

 woven with, but quite independent of, the blood-capillaries. From 

 this network lymphatic vessels pass off, and finally discharge 

 their contents into one or other of the veins. Many of the 

 lower Craniata possess spacious lymph-sinuses surrounding the 

 blood-vessels, and there are communications between the lym- 

 phatics and the coelome by means of minute apertures or 

 stomata. The lymphatics contain a fluid called lymph, which 

 is to all intents and purposes blood minus its red corpuscles. 

 The lymph-plasma consists of the drainage from the tissues : it 

 makes its way into the lymph capillaries, and thence into the 

 lymphatics, which are all efferent vessels, conveying the fluid 

 from the capillaries to the veins. Leucocytes are added to the 

 plasma in bodies, called lymphatic glands, which occur in the course 

 of the vessels. Valves may be present to prevent any flow of 

 lymph towards the capillaries, and in some cases the course of the 

 fluid is assisted by lymph hearts, muscular dilatations in the course 

 of certain of the vessels. The lymphatics of the intestine have an 

 important function in the absorption of fats, and are known as 

 lacteals (Ic.) 



