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COLLEGE ZOOLOGY 



short and blunt and covered with ectoderm (Fig. 133, 1). 

 Around their bases are many whitish modified spines called 

 pedicellarice (Fig. 133, 10). These are little jaws which when 

 irritated may be opened and closed by several sets of muscles. 

 Their function is to protect the dermal branchice (Fig. 133, 5), 

 to prevent debris and small organisms from collecting on the 



Fig. 133. — Diagram of a transverse section of the arm of a starfish. 

 1, ectoderm; 2, jelly; 3, peribranchial space in the skin; 4, peritoneal lining 

 of the body-cavity; 5, a branchia; 6, pyloric caecum; 7, mesentery support- 

 ing a caecum; 8, spine; g, ossicle in skin; 10, pedicellaria; 11, ambulacral 

 ossicle; 12, adambulacral ossicle; 73, radial trunk of water-vascular system; 

 14, radial septum separating the two perihasmal spaces; 75, radial nerve-cord; 

 16, ampulla of tube-foot; 77, tube-foot; 18, perihaemal space; ig, ccelom. 

 (From Shipley and MacBride.) 



surface, and to capture food. The skeleton serves to give the 

 animal definite shape, to strengthen the body-wall, and as a 

 protection from the action of waves and from other organisms. 



The Muscular System. — The arms of the starfish are not 

 rigid, but may be flexed slowly by a few muscle fibers in the 

 body-wall. The tube-feet are also supplied with muscle fibers. 



Ccelom. — The true body-cavity of the starfish is very large 

 and may be separated into several distinct divisions. The 



