CCELENTERATA : SERTULARIDA. 



121 



crust which spreads over shells and other foreign bodies. The tentacles of 

 the nutritive zooids form a single sub-alternate series. The generative 

 buds are produced upon imperfect, non-tentaculate polypites ; and are 

 mere sac- shaped protuberances, enclosing diverticula from the body-cavity, 

 but not detached from the parent organism. 



Fig. 45. Group of zooids of Hydractinia echinata, enlarged. (After Hincks.) 

 a a Nutritive zooids ; b b Generative zooids, carrying sacs filled with ova. 



ORDER III. SERTULARIDA (Calyptoblastica, Allman; The- 

 caphora, Hincks). This order comprises those Hydrozoa 

 " whose hydro soma is fixed by a hydrorhiza, and consists o 

 several polypites, protected by hydrotheccz, and connected by a 

 ccenosarc, which is usually branched and invested by a very firm 

 outer layer. Reproductive organs in the form of gonophores 

 arising from the ccenosarc or from gonoblastidia" (Greene). 



The Sertularida resemble the Corynida in becoming perma- 

 nently fixed after their embryonic condition by a hydrorhiza, 

 which is developed from the proximal end of the ccenosarc ; 



