xiii PHYLUM CHORD ATA 97 



has to be borne in mind that the direction of the current of the 

 blood in the Craniata is exactly the opposite of that in the 

 Annulata. 



The Hood of Craniata is always red, and is specially distin- 

 guished by the fact that the haemoglobin to which it owes its 

 colour is not dissolved in the plasma as in most red-blooded Inver- 

 tebrates, but is confined to certain cells called red Hood-corpuscles 

 (Fig. 776), which occur floating in the plasma in addition to, and 



FIG. 776. Surface and edge views of re$ blood-corpuscles of Frog (A) and Man (B). nu. nucleus. 



(From Parker's Bloloijy.) 



in far greater numbers than, the leucocytes. They usually have 

 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-nucle- 

 ated, and usually circular. The red corpuscles do not perform 

 amo3boid movements. 



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

 up by the hemoglobin. 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 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.y., 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. 773, 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 



