TYPES OF CCELENTERA A MEDUSOID. 145 



buds, and also free medusoid forms, which may be (a) 

 liberated members of a hydroid colony, or (b) independent 

 organisms. Besides these there are complex colonies of 

 medusoid forms (Siphonophora). 



The hydroid type, except in minor details, usually 

 resembles Hydra. In some cases the tentacles are solid, 

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

 two circles, an outer and an inner (e.g. Tubular ia). In 

 some of the hydroid colonies, notably the Millepores and 

 Hydractinia, the polypes 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 

 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. 62, (3)). 



On the convex surface of the bell the ectoderm forms 

 simply an epithelial layer ; on the concave surface it is 

 differentiated into muscle cells on the velum, the manu- 

 brium, and the tentacles, nerve cells at the base of the 

 velum, and stinging cells on the tentacles. The endoderm 

 is ciliated ; it lines the food canal, and extends also into the 

 tentacles. The mesogluea forms a thickened jelly, present 

 more especially on the convex (ex-umbrellar) surface. 



The mouth opens into the canal of the manubrium, which 

 leads to the central cavity of the dome. With this a varying 

 number of unbranched radial canals communicate ; these 

 open into a marginal circular vessel, which communicates 

 with the cavities of the tentacles. Digestion is intracellular, 

 and probably goes on throughout the whole of this "gastro- 

 vascular" system. 



The movements of the bell are caused by the contractions 

 of the ectodermic muscle cells. 



The nervous system consists of a double ring of nerve 

 fibres around the margin of the bell. With these are 

 associated ganglionic cells, which apparently control the 

 muscular contractions. 



10 



