170 



CORALS 



good deal, but as a roiigli guide for comparison with other 

 species, it may be said tliat the average length of a zooecium 

 is about 0-3 mm. 



The zooecia appear to be irregularly disposed with a 

 tendency to overlap and form layer upon layer of super- 

 imposed laminae. The walls of the superficial zooecia are 

 very thin and brittle, and there does not seem to be the same 

 tendency for the walls of the lower layers to become thicker, 

 so that the colony as a whole retains its light spongy texture. 



Fig. 82. — Porella ccniprcssa. From west coast of Scotland. Xat. size. 



The genus Cellepora includes a very large number of 

 species and several of them are represented in British waters. 



One of these, C. pumicosa, is very commonly found 

 attached to the seaweed and zoophytes cast up on the beach 

 after a storm. It has the form of little white, or if fresh, 

 pink dome-shaped encrusting masses of zooecia a quarter of 

 an inch or less in diameter. 



Porella. — This cosmopolitan genus of Polyzoa includes 

 a species of coral, P. compressa, which is not uncommonly 

 brought up in the dredge off the coast of Cornwall, off the 

 west coasts of Ireland and Scotland, and in some other 

 British localities. It seems to be confined to deep water 



