206 ANIMALS 



the body, where they divide into another network of capillary 

 vessels, through which the blood is distributed to all the tissues. 

 From this second system of capillaries larger vessels lead to 

 another large, longitudinal vessel lying between the ventral 

 nerve cord and the intestine. This vessel is connected with the 

 intestinal system of capillaries, and thus a complete circuit is 

 formed. The larger vessels are muscular, and by their rhythmi- 

 cal contraction the blood is forced along. This is especially 

 true of the dorsal longitudinal vessel, in which a continuous 

 series of contractions pass forward from posterior to anterior, 

 forcing the blood along in the same direction. In the ventral 

 longitudinal vessel the blood flows from anterior to posterior. 



471. In the system just described the vessels are "closed"; 

 that is, they do not open into the body cavity, and they con- 

 tain blood, which is not the same as the body fluid. In many 

 invertebrates, however, the body cavity forms a part of the 

 system of spaces through which the blood circulates and in this 

 case there is no distinction of blood and body fluid. This is 

 the type of circulatory system found in Crustacea and Insects. 

 A part of the dorsal vessel is much enlarged and very muscular. 

 By its contraction the blood is forced forward and backward 

 through branching vessels to all parts of the body. On its 

 return the blood enters large spaces, which represent the body 

 cavity and thus it reaches a space immediately surrounding 

 the heart, the pericardial cavity. The heart is pierced by six 

 or eight pairs of openings guarded by valves. When the heart 

 expands the blood enters by these openings (ostia), but when it 

 contracts the closing of the valves prevents the return of the 

 blood through the ostia. It is therefore forced out through the 

 vessels. (See Fig. no.) 



472. In Vertebrates the circulatory system is always closed 

 and the heart is developed into a powerful pumping organ with 

 two, three or four chambers. From the intestine the absorbed 

 food is first carried to special organs in which it undergoes 



