302 



PHYLUM ECHINODERMA 



are wholly ventral, and the apical area seen on the dorsal 

 surface of the young forms is not demonstrable in the adult. 

 On the shell th6re are obviously many spines, most 

 abundant on the inter-ambulacral areas. Their bases fit 

 over ball-like knobs, and are moved upon these by muscles. 

 But besides these, there are modified spines — {a) several 

 kinds of pedicellarias, with three snapping blades on a 



Fig. 164. — Dissection of sea-urchin. 



M. at the lower pole is the mouth ; M. at the upper pole is the madreporic 



plate ; T.T., one of the large tentacular tube-feet around the mouth ; 



■ S.G., a skin-gill ; ST., a standard or perignath ; AL., an alveolus ; 



R.V., a radial vessel, with ampullae {A.) ; intestine {Int.) fixed by 



mesenteries 

 tube-feet. 



P., a pedicellaria ; G., a gonad ; SP., spines ; T.F. 



mobile stalk, sometimes with apical glands ; and (b) small 

 globular sphaeridia, which show some structural resem- 

 blances to otocysts. It is said that, like true otocysts, 

 they are concerned with the perception of direction of 

 motion. New spines and pedicellariae can be grown to 

 replace those that are shed in unwholesome conditions or 

 rubbed off by accident. This is the only marked regenera- 

 tion in sea-urchins. 



