ii 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 



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

 Hickson. The Cambridge Natural History, Vol. I. 

 Delage and Herouard. Zoologie concrete, Tome II. 



II. SYSTEMATIC WORKS FOR THE IDENTIFICATION OF SPECIMENS 



Hincks. British Hydrozoa. 

 Mayer. Medusae of the World. 

 Gosse. Actinologia britannica. 



