ii THE METAZOA 71 



walls of the blood-vessels themselves, or by the agency of a 

 special organ, the heart. The heart is essentially a sac with 

 muscular walls. Its cavity is in communication with the 

 main blood-vessels, and its walls contract regularly, and 

 drive the blood through the system of vessels, the direction 

 of flow being regulated by a system of valves. 



The nitrogenous waste-matters which are produced as a 

 result of the chemical changes that accompany vital action 

 in the various organs, are separated out and got rid of by a 

 system of organs known as the organs of excretion or renal 

 organs this process of elimination being known as the 

 process of renal excretion. 



It is by means of the nervous system that the animal 

 receives impressions from the exterior and from the internal 

 organs, and that the various internal parts are brought into 

 vital communication with one another. The nervous system 

 extends throughout the body as a complicated system of 

 nerves or bundles of nerve-fibres. Large aggregations of 

 nerve-cells and nerve-fibres forming the centres of the 

 system are known as nerve-ganglia. When one of these, 

 or a group of them, situated towards the anterior end, 

 preponderates in size over the others, it is termed the 

 brain. Forming an important part of the nervous system 

 are the organs of the special senses sight, hearing, smell 

 and taste, each of which is an organ adapted for the 

 reception of impressions of a special kind from the ex- 

 terior, the impressions of light, of sound waves, of the 

 particles and substances that produce the sensations of 

 smell and taste. The less specialised sense of touch and 

 of heat and cold is diffused generally over the integument, 

 in which there are frequently special cells, or groups of 

 cells, with nerve-fibres terminating in or around them, that 

 are concerned with such sensations. 



