GENERAL ZOOLOGY 



Growing 

 tip 



Mouth 



Mouth 



Anus 



"Hearts" 



Capillaries 



Fig. 17.2. Schematic diagrams of circulatory mechanisms in invertebrates. A, circulation 

 within a coelenteron, extending to all parts of the body (Coelenterata). B, blood vessels 

 without pulsatile walls, the blood being driven back and forth by movements of the body 

 (Nemertinea). C, blood vessels extensively developed, the larger ones being pulsatile and 

 maintaining a definite circulatory course of the blood (Annelida). 



The true heart, which is found in all well-developed circulatory systems, 

 probably arose in evolution through localization and specialization of the con- 

 tractile functions of a major blood vessel. 



Among well-developed invertebrate circulatory systems, two major types 

 can be distinguished. In one, called the closed system, the blood is en- 

 closed within vessels that are continuous through a circuit of heart, arteries, 

 capillaries, and veins. Such a system is found in many annelids, and in 

 cephalopod mollusks such as the squid (see Fig. 13.19, p. 391). More com- 

 mon among invertebrates is the second type, the so-called open system, ex- 

 emplified in pelecypod mollusks (see Fig. 13.3, p. 374). In these animals 

 blood flows from the heart through arteries to all parts of the body; leaving 

 the smallest branches of the arteries, it passes not into capillaries but into 

 intercellular spaces of the mesh-like mesenchymal tissue, collectively called 

 sinuses. From these sinuses blood enters the smallest branches of the veins 

 and so is returned to the heart. 



Arthropods such as the crayfish have a similarly open system, in which 

 the sinu.ses form large perivisceral cavities constituting the hemocoel. Such 



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