COELENTEEATA 



43 



Medusoid 



Radial canals 



Circular canal 



Ectoderm 



Endoderm 



Plaiiula 



Young stages 



FIG. 18. Obelia geniculata. Diagrams to illustrate (a-c) the development of 

 the medusoid from the blastostyle, after Kiihn. A plan (d) is also given 

 of the developing medusoid viewed from the sub-umbrella surface, after 

 Hadzi. The free medusoid (e) is shown and a vertical section (/) to indicate 

 more exactly its structure. Larvae are developed in the female medusoid 

 after the eggs are fertilised and they are called planulae. They are freed, 

 and after a short period of freedom settle down and grow each into the 

 hydroid. 



