COELENTERATA 



129 



Others they may even be united in the same species and the same 

 colony of that species. A survey of the phylum is very largely con- 

 cerned with the variations of these types and the combination of them 

 in the life histories of the different coelenterates. 



The polyp (Fig. 109 A) is an attached cylindrical organism with a 

 thin body wall consisting of two single layers of ectoderm and endo- 

 derm separated by a narrow structureless lamella. At the free end an 



Fig. 108. Diagram of Hydra to show the nervous net. n.c. nerve cells. 



oral cone occurs and at its apex the mouth opening into the enteron. 

 The oral cone (in the Hydrozoa) is surrounded by a number of 

 tentacles, which are usually very extensible and armed with batteries 

 of nematocysts, by which the living animals, on which the coelenterate 

 feeds, are caught. Tentacles contain a prolongation of the endoderm 

 which may form a tubular diverticulum of the enteron or a solid core. 

 The medusa (Fig. 109 C) is a free-living organism differing from the 



