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EVOLUTION OF ANIMALS 



Part V 



Their arms are easily snapped off and in turn easily regrown (Figs. 32.1, 

 32.9). 



Class Echinoidea — Sea Urchins, Sand Dollars 



Sea urchins are generally biscuit-shaped with more or less prominent spines 

 for which the class is named. Sand dollars are flat, cooky-shaped, with very 

 short usually fine spines. Instead of the separate pieces of skeleton being 



Spines 



Pedicellariae 



Gills 



Pedicellariae 



A 



B 



Muscles 



Fig. 32.3. A and B, small portion of the surface of a star fish showing the large 

 spines and finger-shaped skin gills through whose thin walls gases are diffused, 

 oxygen into the tissue fluid which they contain and carbon dioxide from tissue 

 fluid into the surrounding water. The minute pincers or pedicellariae cooperate in 

 keeping the surface clean aided by cilia which create currents of water. C, a single 

 pedicellaria. These pincers are very responsive to touch. Hundreds of them will 

 snap and clamp if a hair is drawn across the body. (After Jennings. Courtesy, 

 Fasten: Introduction to General Zoology. Boston, Ginn and Co., 1941.) 



embedded in a muscular body wall as in starfishes, these skeletons are in- 

 flexible cases formed of limy plates lightly fused together (Fig. 32.1). The 

 spines are attached by ball-and-socket joints and sea urchins walk on them as 

 if on stilts. Colonies of sea urchins cling to wave-washed rocks. Sand dollars 

 commonly lie half burrowed in sand rich in organic matter. Relatively few tube 

 feet touch the surface when a sea urchin walks over flat places, but it uses feet 

 from every surface of the body when it climbs (Fig. 32.1). The crystal clear 

 eggs and developing embryos of sea urchins and sand dollars are among the 

 most famous subjects of embryological investigations. 



Class Holothuroidea — Sea Cucumbers 



Some sea cucumbers are replicas of pickled cucumbers; others are long and 

 slender, translucent and beautiful (Fig. 32.1). They have no skeletal frame. 

 What skeleton there is consists of smaU limy plates, helplessly isolated and 

 embedded in the thin muscular body wall. Sea cucumbers rest and travel 

 on their sides mouth forward; all other echinoderms except the crinoids travel 

 mouth down. Superficially sea cucumbers seem bilaterally symmetrical, and 



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