146 ANNELIDA fen. 



hungry for oxygen, the oxy-haemogiobin is decomposed : the reduced 

 haemoglobin is purplish in colour. At the same time the cell gives 

 up carbon dioxide to the blood. The relations of this gas in the 

 blood are less understood than those of the oxygen, but like the 

 latter it is in loose chemical union, though not with the haemo- 

 globin. * In Vertebrate animals the sodium of the blood provides 

 the means of conveying the carbon dioxide to the respiratory organs. 

 When the blood again approaches the skin carbon dioxide is got rid 

 of, oxy-haemoglobin being again formed by fresh oxygen taken in. 



The earthworm is the first animal which we have so far studied 

 in which there is any flow of the blood in a definite direction in 

 a word a circulation. Movements of the fluid contained in the 

 vessels of the Nemertine worms in various directions are doubtless 

 occasioned by the contractions of the body muscles of those long 

 thin animals in the Rotifera owing to their small size the whole 

 bulk of the fluid is so small that the molecular movements of 

 diffusion are enough to thoroughly mix it up. The direction of 

 movement of the blood in the dorsal vessel in the Earthworm can 

 be seen through the skin. Waves of contraction are seen to begin 

 in the hinder end of this vessel and to travel forwards, pushing 

 the blood in front of them. In the other vessels the direction of 

 the flow can be inferred from the existence of valves. These are 

 flaps projecting inwards from the walls of the vessels into their 

 cavities. They are arranged in pairs so as to open only in one 

 direction. When they swing in the opposite direction they come 

 together and close the cavity of the vessel. The blood must there- 

 fore flow in the direction in which the valves open. In the dorsal 

 vessel there is in each segment a pair of valves opening forwards. 

 Where the parietal vessel joins the dorsal vessel there is a pair 

 of valves opening upwards, and lastly in each heart there is at 

 each of three different levels a pair of valves opening downwards. 

 Hence we infer that as blood flows forwards in the dorsal vessel, 

 blood pours into it from the parietal vessel which has ascended 

 from the sub-neural vessel along the outer side of the body. Some 

 of this blood has doubtless passed into the dermal plexus, as the 

 network of fine vessels is termed which underlie the skin and are 

 connected with the parietal vessel. In this plexus the blood has been 

 recharged with oxygen. Blood passes from the dorsal vessel down- 

 wards into the ventral vessel through the hearts, and as this latter . 

 vessel is devoid of muscles and valves it is to be regarded as a reservoir 

 in which the blood is stored and from which it passes out in all 



