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GENERAL BIOLOGY 



the same route. Such a system in its most elemen- 

 tary form may be found in the insects. Here it 

 consists of a simple dorsal tube open at both ends, 

 which by its alternate contractions and expansions 

 keeps the body fluids " stirred up " and provides for 



FIG. 43. Diagrams of the circulation of the frog (A) and of the 

 lobster (B), the former illustrating a closed system, the second, an 

 "open" system: L, left auricle; R, right auricle; V, ventricle; 1, 

 arterial branch to the head ; 2, arterial branch to the rest of the body ; 

 3, arterial branch to the lung (P) ; 4, pulmonary vein returning aerated 

 blood from the lung to the left auricle ; 5 and 6, venous trunks return- 

 ing blood from the head and body to the right auricle ; H, heart sending 

 out arterial blood both to the anterior and to the posterior part of the 

 body ; s, sinuses, in which the blood accumulates to be returned through 

 the gills (G) to the pericardial chamber (p.c.), whence it finds its way to 

 the interior of the heart through the ostia (o). 



an indefinite circulation of substances. In the sides 

 of the tube are mouthlike apertures provided with 

 valves, called ostia. Through these the body fluids 

 can enter when the tube expands, but, owing to 

 the valves, they can leave only through the open 

 ends. The necessity for a complicated set of blood 

 tubes in these animals is obviated by the develop- 



