HYDROIDS AND MEDUSOIDS 



l8l 



instead of hollow as in Hydra, and they may be arranged in 

 two circles — an outer and an inner {e.g. Tubularia). In 

 some of the hydroid colonies, notably the Millepores and 

 Hydr actinia, the polyps are very dissimilar to one another, 

 and have become specialised for the performance of different 

 functions. 



The medusoid type is like an inflated hydroid adapted 



Fig. 90. — Bougainvillea. — Alter AUman. 



A. A small piece of a hydroid colony, p., Perisarc ; m., medusoid 



bud ; h., hydranth or polyp head. 



B. A medusoid. ma., Manubrium; r.c, radial canal; s., sense 



organ. 



for swimming. It is bell-shaped, and down the middle of 

 the bell hangs a prolongation — the manubrium — which 

 terminates in the mouth. Around the margin of the bell 

 there is a little shelf, the velum or craspedon, which projects 

 inwards, and is furnished with muscle cells. The margin of 

 the bell also bears tentacles, usually hollow, and abundantly 

 furnished with stinging cells (Fig. 83, 3). 



