DEVELOPMENT. 3 ! 7 



bands now grow towards one another, so that the vent conies to lie beneath 

 them (stages 7 and 8). They also join themselves to the two upper bands, so 

 that there is formed a single zone of cilia, which persists to the end of the 

 larva's life. Already can be distinguished the beginnings of the apex and of the 

 processes (e), which finally lengthen into the arms that give such a strange 

 appearance to the larva, in the sea-urchins and also in the star-fish and brittle- 



DEVELOPMENT OF A SEA-URCHIN (stage 10). 



stars. In stage 8 can be seen, at 6, the pore that admits water to the water- 

 vascular system; and at this point will lie the madreporite of the future sea- 

 urchins. The next illustration shows all these parts in a rather more advanced 

 stage : a is the vent; c, the hinder intestine ; d, the stomach, around which a deposit 

 of spicules indicates the first beginnings of the body of the sea-urchin; o, the gullet; 

 m, the mouth ; e, the arms of the larva : r, the calcareous rods that support them ; v, 

 two more strongly-developed and slightly projecting portions of the ciliated band ; 

 and w, the water-vessels. The larva in its full development is shown in the 



