72 INVERTEBRATE ZOOLOGY 



of two layers of cells, the ectoderm and the entoderm. The most 

 distant cells of such an organism are so slightly removed from 

 the store of digested food material that distribution is by direct 

 transfer from cell to cell. In the medusae of the same phylum, 

 the digestive sac is relatively small and located near the center of 

 a much enlarged body within which transfer from cell to cell is not 

 so readily possible. Under these conditions a distributive system 

 arises, intimately associated with but somewhat distinct from 

 the digestive system. The tubes or pouches of such a gastro- 

 vascular system mark the beginnings of a separate system for 

 circulation. 



Among the coelomate animals, digested food materials fre- 

 quently pass through the wall of the digestive tract into the 

 coelomic cavities where it becomes recognizable as the body 

 fluid. Only in the simple coelomate animals is this fluid entirely 

 free within the body cavities, for it becomes more or less confined 

 within a system of definite channels comprising the vessels of the 

 circulatory system. When the fluid is continuously within 

 vessels, the system is termed a closed system, but when in any 

 part of its course the fluid is emptied into the body cavity or into 

 sinuses the system is designated as an open circulatory system. 

 Muscles become associated with at least part of the vessels which 

 then function as a pumping organ or heart. In many inverte- 

 brates, the most conspicuous pumping organ is the vessel, or 

 part of the vessel, which lies dorsal to the digestive tract, but 

 hearts are frequently located in other regions or the body, as, for 

 example, the gill hearts of molluscs and the modifications of the 

 circular vessels in the annelids. 



In some instances, notably in the molluscs, the heart becomes 

 differentiated into various chambers. Those which receive the 

 blood are termed auricles and those which force the blood out 

 into the vessels are called ventricles. The heart presents an 

 interesting instance of independent or parallel development in 

 invertebrates and vertebrates. A three-chambered heart is 

 fairly common in the molluscs, while the fishes have but a two- 

 chambered heart. 



Respiratory System. — The energy manifested by a living 

 organism results largely from the union of oxygen with proto- 

 plasm and its contained food substances. Highly complicated 

 organic compounds are oxidized or broken down into simpler 

 waste substances, thereby transforming the potential energy of 



