76 



characteristic feature. The mesogloea may be a thin sup- 

 porting lamella, or it may form an extensive mass, and it 

 often contains^ migratory cells. In the simpler types (e.g., 

 Hydra) the ccelenteron is a plain tube ; in higher forms 

 (e.g., Sea-Anemone) it has a gullet or stomodoeum, formed 

 by an intucking of ectoderm at the oral end, and it is also 

 divided into compartments by radial septa or mesenteries, 

 folds of the wall which extend towards the centre. 



There are two chief structural types, the tubular polyp 

 and the expansive bell-like medusa. Both types are often 

 present in the course of a life- history and illustrate alter- 

 nation of generations. 



In the polypoid types the ectoderm often secretes a horny 

 protective covering, the perisarc, and in certain forms a 

 deposition of lime results in the formation of corals. 



Multiplication by budding (gemmation) is common and 

 results in colonies which show division of labour which in 

 certain instances amounts to polymorphism, more than 

 two different kinds of " persons " being produced. The 

 mode of reproduction is sexual. 



Most Ccelenterates are marine, a few (e.g., Hydra) occur 

 in freshwater. Commensalism is frequent (e.g., Sea- A nemone 

 and Hermit- Crab). 



CLASS HYDROZOA. 

 Hydra (the Freshwater Polype). 



Where is Hydra found ? Describe it. 



Upon water weeds in ponds. It has a thin tubular body 

 with six or more hollow prolongations, the tentacles, at 

 one end and a disc or " foot " at the other. The tentacles 

 surround an oral cone at the top of which is the mouth. 

 The body is about a quarter of an inch in length, the ten- 

 tacles are often longer ; and the colour is green, brown, 

 or white according to the species. 



