9-i ZOOLOGY SECT. 



between the blood-vessels of Craniata and those of the higher 

 Worms. The sub-intestinal vein, heart, and ventral aorta together 

 form a ventral vessel, the dorsal aorta a dorsal vessel, and the aortic 

 arches transverse or commissural vessels. The heart might thus be 

 looked upon as a portion of an original ventral vessel, which has 

 acquired strongly muscular walls, and performs the whole function 

 of propelling the blood. But in making such a comparison it 

 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 blood of Craniata is always red, and is specially distinguished 

 by the fact that the haemoglobin to which it owes its colour is not 

 dissolved in the plasma as in most red-blooded Invertebrates, but 

 is confined to certain cells called red blood-corpuscles (Fig. 785), 

 which occur floating in the plasma in addition to, and 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-nucleated, and 

 usually circular. The red corpuscles 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 



FIG. 785. Surface and edge views of red SCarletColoUT,butaSSUmesabluish- 

 blood-corpuscles of Frog U) and Man purple hue after giving Up its 

 (B). nu. nucleus. (From Parkers .-, . . r 



Biology.) 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.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. 782, 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 ccelome 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 



