140 



AN INTRODUCTION TO ZOOLOGY 



FIG. 65. Part of .a colonial Hydrozoan, Obelia (A), and a free-swimming 

 medusa (B) from another hydroid colony (Bougainmllia). i, ectoderm ; 

 2, entoderm ; 3, mouth ; 4, coelenteron ; 5, coenosarc ; 6, perisarc ; 

 7, hydrotheca ; 8, blastostyle ; 9, medusa bud ; 10, gonotheca. (From 

 Shipley and MacBride.) 



