SEA URCHIN. 



25 1 



pair of pores a tube -foot is connected with an internal 

 ampulla. In the star-fish the ambulacral areas 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 there are obviously many spines, most 



a 7?i 



FIG. 120. Diagram of sea-urchin (Echinus}. After Huxley, 



slightly modified. 



m., Mouth ; -., gut cut through, and with coils omitted ; a., anus ; 

 ma., madreporite \sf., stone canal ; e.c., circular canal ; P., one of 

 the Polian vesicles; r.r., racial vessel; am., ampulla of tube- 

 foot ; tf., tube-foot ending in sucker ; n. , radial nerve given off 

 from nerve-ring; a!., alveolus, one of the parts of Aristotle's 

 lantern, at the left the alveolus is removed to show one of the 

 strong teeth (/.) ; f., falces, to which the retractor muscles (m.) 

 of the lantern are attached ; sp. , spines on surface of test ; pe., 

 pedicellaria ; /., flesh}- lobes or lips ; oc. , one of ocular plates. 



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 two modified forms of spines- 

 (a) the minute pedicellariae, with three snapping blades on 

 a soft stalk, and sometimes with apical glands ; and (ft) small 



