302 THE MAINTENANCE OF THE INDIVIDUAL 



a system of sinuses, through the gills to the pericardial sinus surround- 

 ing the heart. It passes into the heart by means of a series of openings 

 called ostia, guarded by one-way valves. 



Insects, a still more highly specialized group, have a very direct 

 respiratory system called a tracheal system, which takes over the job 

 usually handled by the blood stream, bringing the oxygen directly to 

 the tissues through a network of tubules, or tracheae. This has been 

 discussed previously in detail (pages 209-210). 



Closed Circulatory Systems 



Among Invertebrates 



Systems of this general type are found in a large and diversified 

 group of organisms beginning with the invertebrates and extending 

 throughout the vertebrate group. The motive power of such cir- 

 culatory devices consists essentially of a central pumping plant or 

 heart, from which extends a series of arteries that break down into 

 minute capillaries in the tissues and then pass into gradually larger 

 vessels known as veins which return the blood to the heart. Some- 

 where in the capillary circuit the blood is aerated, giving off carbon 

 dioxide and taking in oxygen. The earthworm furnishes an example 

 of such a system in the invertebrates. 



Among Vertebrates 



In all of the vertebrates there is a well-developed closed type of 

 circulatory system, although the supplementary lymphatic system 

 might be construed as a sort of open system. In order to understand 

 the work performed by these systems we must turn our attention to 

 the various component parts involved and consider their functions. 



The Blood 



Blood is a red fluid which, examined microscopically, is seen to be 

 composed of three types of corpuscles, red and white, circulating in a 

 liquid plasma, and the much smaller blood platelets. The first con- 

 tains hemoglobin, which combines with oxygen in a loose combination 

 forming oxyhemoglobin , useful in respiration. The white corpuscles, 

 on the other hand, are the scavengers of the body. They are ame- 

 boid in shape and are concerned, in part at least, with the defense 

 of the body against bacterial invasion. Under certain stimuli great 



