THE PHYLUM ECHINODERMATA 



principally active, and carry the products of digestion into the caeca for 

 absorption. 



The coelomic fluid contained in the extensive coelomic body cavities (Fig. 

 16.4) performs the functions of a circulatory system in the starfish. This 

 colorless fluid contains many phagocytic, amoeboid cells and, very much 

 like the hemocoelic blood of an arthropod, bathes the tissues of the body. In 

 the starfish, however, there are no functional blood vessels, and there is no 

 pulsatile heart. Circulation of the coelomic fluid is brought about by the 

 action of flagella borne on the cells of the peritoneal lining. This single- 

 layered epithelium covers both the inner surface of the body wall and the 

 outer surfaces of all the organs. It is composed of cuboidal or flattened cells, 

 each of which bears a single long flagellum. These flagella together maintain 

 specific currents which prevent stagnation of the fluid and facilitate exchange 

 of materials between the fluid and the tissues. Nutrients are passed into the 

 coelomic fluid by the cells of the pyloric caeca, for transport to all other 

 parts of the body; oxygen diffuses into the fluid through the thin-walled, 

 finger-like papulae of the body wall and is carried in simple physical solu- 

 tion. Carbon dioxide and soluble nitrogenous excreta are transferred to the 



Spine 



Pyloric caecum 

 Papula 



Lateral 

 canal 



Radial 



perihaemal 



canal 



Interossicular 

 muscle 



Radial 

 nerve cord 



Longitudinal 

 muscle layer 



Pedicellaria 

 Podium 



Sucker 



Fig. 16.4. Diagrammatic cross section of an arm of Asterias. Note that tlie genital pores, 

 included in this drawing to show general relationships, actually lie very near the proximal 

 end of the arm, in the angle between adjacent arms. (Redrawn, after H. C. Chadwick, from 

 W. M. Reid in F. A. Brown, Jr., et ai. Selected Invertebrate Types, copyright 1950 by John 

 Wiley and Sons, Inc., printed by permission.) 



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