IV LUMBRICUS 



141 



extremely close relations with the living protoplasm of the various organs 

 of the body. The capillary network is well seen in the wall of the ali- 

 mentary canal, immediately outside the endoderm, where it is concerned 

 with the taking up of the products of digestion, in the wall of the 

 nephridium where it is concerned with excretion, and immediately 

 beneath the epidermis where it is concerned with respiration. From 

 the capillary network the blood drains away into small vessels which 

 uniting together in a branched system return it eventually into the main 

 vessels. 



The blood is propelled onwards by waves of peristaltic contraction 

 of the walls of the larger vessels. These are particularly accentuated 

 in the hearts, in which the peristaltic waves pass downwards from dorsal 

 to ventral end. The hearts being situated towards the head end of the 

 worm it follows that the blood-stream passes forwards in the dorsal 

 vessel, ventralwards in the hearts, and tailwards in the ventral vessel. 



The blood itself consists of small irregular or rounded cells floating 

 in a copious fluid or plasma. The latter is coloured " blood-red " owing 

 to its carrying in solution the same iron-containing colouring matter- — 

 haemoglobin — as gives the red colour to the blood of Vertebrates. 

 This substance haemoglobin is intimately concerned with the process 

 of respiration. It has a great affinity for oxygen and if brought into 

 relation with it at once combines with it to form oxyhaemoglobin 

 characterized by its bright red colour. The oxygen and the haemo- 

 globin in this compound are combined in a very loose fashion and are 

 readily torn apart. It is this chemical characteristic that enables the 

 haemoglobin to perform its great physiological function, that of acting 

 as a vehicle for the oxygen so necessary for the metabolism of all the 

 living protoplasm of the body. As the blood circulates through the 

 capillary network of the skin the haemoglobin combines with the oxygen 

 which diffuses in from the outer air. The oxyhaemoglobin so formed 

 is then whirled away in the blood-stream until, somewhere in the interior 

 of the body, coming into the neighbourhood of tissues hungry for 

 oxygen, it breaks up, sets free its oxygen, so that it can be appropriated 

 by the tissue, and passes onwards as reduced haemoglobin until it under- 

 goes re-oxygenation on again passing near the surface of the body. 



There finally remains to be mentioned the nervous system which 

 serves to control the activity of the worm and to knit together its con- 

 stituent parts into a coherent and functional whole. The most important 

 change which we see when we compare higher stages in the evolution 

 of the nervous system with lower consists in the greater centralization 

 of control, correlated with greater concentration of ganglion-cells, so 



