OUR BEST FEATURES 



zoologists, who infer that the greatly simplified 

 form Amphioxus (Fig. 54), together with all the 

 vertebrates, represent offshoots of some still undis- 

 covered stock that also gave rise to the acorn- 

 worms (Balanoglossus) , the starfishes and certain 

 other peculiar groups. According to this view. 



Fig. 55. Larv^ of Echinoderms: Sea-cucumber (A), Starfish 



(B) AND OF THE "AcORN-WORM" (aFTEH DeLAGE AND H^ROUARd). 



For details, see p. xxv. 



the common ancestors of all these diversified 



groups were exceedingly simplified, free-swimming, 



marine organisms, consisting chiefly of a digestive 



tube bent at a right angle and enclosed in a thin 



balloon-like tissue, more or less folded into plaits 



and provided with strips of cilia, by the lashing 



of which the floating bag moved slowly through 



the water. Such forms (Fig. 55) are found living 



today as the larvae or young stages of starfishes, 



93 



A 



