INTRODUCTION. 



XX111 



stage of their development they liberate themselves from 

 the parent stock, escape from the sac which had hitherto con- 

 fined them, and enter on a term of independent existence. 

 (Woodcut, fig. x.) 



Fig. x. 



a. 



a. The gouozooid enclosed in its ectotheca. b. The same, after the rupture 

 of the sac, on the point of detaching itself. 



They are now altogether separated from the colony to 

 which they belong, and so thoroughly are their affinities 

 concealed by the locomotive and other adaptive organs 

 with which they are furnished, that they might readily 

 pass for members of another tribe. Indeed we can 

 scarcely imagine a more complete contrast to the staid and 

 stationary zoophyte, in outward form and habit of life, than 

 tlic medusiform zooid which it evolves from its own sub- 



