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ANIMALS WITH SPINY SKINS— THE ECHINODERMS 



The starfish is well protected by a skeletal system. The outer sur- 

 face of the body is covered by skin through which hard knobs of lime 

 protrude to form spines. This gives excellent protection and support 

 for the soft body parts within the body. At the same time the arms 

 are quite flexible because of the soft skin between the spines which 

 allows the arms to bend easily. There are small pinchers, the pedicel- 

 lariae, which come out from around the base of the spines. These 

 pinchers keep off foreign matter that might adhere to the body of the 

 starfish and interfere with respiration and other life processes. 



Photo by Winchester 



Fig. 20.1. A starfish in an aquarium. Note the suction cup appearance of the ends 

 of the tube feet where they are attached to the glass. 



In the living starfish there are delicate little finger-like projections 

 coming out from the skin between the spines. 



These projections are extensions of the body cavity, or coelom, which 

 contains the coelomic fluid. They are called skin gills and absorb 

 oxygen from the water and return the carbon dioxide to it in respira- 

 tion. The coelomic fluid circulates all over the body and takes care of 

 circulation, but there are no well-organized vessels as there were in the 

 earthworm. The skin gills also aid in excretion. The coelomic fluid 

 contains amoeboid cells which engulf the excretory wastes which diffuse 

 out into this fluid from the body. Eventually, these cells become filled 

 with the waste and squeeze their way out of the body through the skin 

 gills, carrying themselves and the waste out into the water never to 

 return. 



The starfish has a muscular system which enables it to move the arms, 

 to pinch with the pedicellaria, and to make certain other simple move- 



