SEA- URCHINS. 



61 



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Fig. 67. — Bipinnaria with the star- 

 fish budding: from it. e, e', d', g, 

 g\ protuberances of the body- 

 comparable with the "arms" of 

 the Brachiolaria figured in the 

 adjoining- engraving-. 6, mouth; 

 o, vent of the larva; A, germ of 

 the star-fish; 7i, cihated digestive 

 tract; i, ambulacral rosette (germ 

 of the water-vessels). 



Fig. 68. —Brachiolaria of As- 

 terias vulgaris, enlarged, 

 with the star-fish (r) devel- 

 oping at the aboral end. 

 €, median anal arm ; e^, odd 

 terminal oral arm;/, brach- 

 iolar arm; /, branch of wa- 

 ter-tube (icv?') leading into 

 /' odd brachiolar arm; /'", 

 surface warts at base of odd 

 brachiolar arnt 



Class III. — Echikoidea {Sea-urchins). 



General Characters of Sea-TTrchins. — A good idea of the 

 general structure of sea-urcliins may be obtained by an ex- 

 amination of the common sea-urchin, Echinus (Fig. 69), 



Fig. G9. —The common Sea-urchin, Echinus drohachiensis. d. framework of 

 mouth and teeth seen in front; c, the same se^n sideways; a, 6, side and ex- 

 ternal view of a single tooth (pTramid); all natural size. 



