The Circuhfory System - 319 



HEART 

 OSTIA 

 PERICARDIAL CHAMBER 



.STERNAL SINUS 

 .BRANCHIOPERICARDIAL CANAL 

 .GILL VESSELS 

 .MOUTH 

 .GREEN GLAND 



Fig. 17-1. Hemolymph circulation of the lobster, as shown in a diagrammatic longitudinal section. 



ostial valves close, and the hemolymph is 

 forced out through the arteries to the various 

 parts of the body (Fig. 17-1). The arteries 

 convey the hemolymph directly to the tissue 

 spaces in each organ of the body. Some ar- 

 teries go to the enteron and here the hemo- 

 lymph passes through the channels in the gut 

 wall, absorbing food substances as it flows. 

 Large arteries also go to the excretory organs 

 (green glands), and while the hemolymph 

 filters through these organs, it gives up excre- 

 tory wastes collected in other parts of the 

 body. In the muscles and other tissues of the 

 body, the arteries bring the hemolymph di- 

 rectly to the tissues, where it flows out into 

 the tissue spaces and so comes into direct 

 contact with the cells. The hemolymph pro- 

 vides the cells with food and oxygen and 

 carries away the waste products of metabo- 

 lism. 



In returning to the heart the hemolymph 

 of the lobster drains from the tissue spaces 

 into the sternal sinus (Fig. 17-1), a large 

 channel that leads to the gills. Then, in flow- 

 ing through the gills, the hemolymph takes 

 on oxygen from the water bathing the gills. 



and gives up carbon dioxide. Finally, the 

 hemolymph passes via six pairs of broad 

 channels, the branchiopericardial canals, 

 back to the pericardial cavity, completing the 

 circuit. 



INDIRECT (BLOOD-LYMPH) CIRCULATIONS 



The disadvantage of such direct types of 

 circulation is the relatively slow rate of flow. 

 Inevitably the flow is slow because the hemo- 

 lymph must be forced through irregular 

 tissue spaces, where it encounters large re- 

 sistance. This difficulty has been circum- 

 vented in higher animals by the evolution of 

 two circulatory fluids: the blood and the 

 lymph. In vertebrates, for example, the 

 lymph flows slowly through the tissue spaces; 

 but the blood flows at high speed through 

 well-defined pipelike blood vessels. 



In such indirect types of circulation, sub- 

 stances in the blood can reach a tissue cell 

 only indirectly, by diffusing through the wall 

 of a capillary and diffusing through the 

 lymph to the cell in question. However, each 

 tissue of the body is very thoroughly per- 



