11 COELENTERATA 117 



stop. If the umbrella is cut into eight sectors each with an uninjured 

 nerve centre the eight sectors go on pulsating but they soon " lose 

 step " with one another owing to the loss of continuity of the nerve 

 plexus which linked them together into a single co-ordinated whole. In 

 the Hydromedusae the nerve centre forms a continuous ring. In the 

 polyp type of coelenterate with its less complex structure we do not 

 find either definite sense organs or definite nerve centres^ although in 

 the Anemones an approach to the development of nerve centres is 

 expressed by the tendency for ganglion-cells to be especially numerous 

 in the region of the mouth and tentacles. 



BOOKS FOR FURTHER STUDY 



I. General Text-Books 



Sedgwiek. Student's Text-Book of Zoology, Vol. I. 

 Hlckson. The Cambridge Natural History, Vol. I. 

 Delage and H^rouard. Zoologie concrete, Tome II. 



II. Systematic Works for the Identification of Specimens 



Hincks. British Hydrozoa. 

 Mayer. Medusae of the World. 

 Gosse. Actinologia britannica. 



12 



