OTHER CGELENTERATA 



275 



Orqanization of an ObeJia Colony. — Obelia is representative in 

 this particular. The colony consists of three types of individ- 

 uals: (1) hydranths, or feeding individuals (Fig. 133 A), (2) blas- 



FiG. 133. — Alternation of generations in a hydroid such as Obelia. 



A, portion of an Obelia colony. B, development of colony following attachment of 

 planula {PL 2). 



a, zygote; 6, two-cell stage; 6?, blastostyle; 6ia, blastula; c, four-cell stage; /i, hydranth; 

 h.b., hydranth bud; m, medusa; m.b., medusa bud; o, ovum; PL 1 and PL 2, 

 or ciliated larvse with ectoderm and endoderm (c/. Fig. 132 D). y^{\ \ P 



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luj L I B R A R Y 



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