SEA- WORMS AND SEA-ANEMONES 81 



The mere sand-flat of the low tide is not a bad 

 hunting ground ; but the rock pools, often exposed when 

 the tide is out, and the fissures in the rocks and the 

 under surfaces of slabs of rock revealed by turning them 

 over are the greatest sources of varied delight to the 

 sea-shore naturalist. It is well t9 take a man with you 

 on to these rocks to carry your collecting bottles and 

 cans, and to turn over for you the larger slabs of loose 

 stone, weighing as much as a couple of hundredweight. 

 The most striking and beautiful objects in these rock 

 pools are the sea-anemones (Fig. 6 and Frontispiece). 

 They present themselves as disk-like flowers from i to 5 

 inches in diameter, with narrow-pointed petals of every 

 variety of colour, set in a circle around a coloured centre. 

 The petals are really hollow tentacles distended with sea- 

 water, and when anything falls on to them or touches 

 them they contract and draw together towards the centre. 

 The centre has a transverse opening in it which is the 

 mouth, and leads into a large, soft-walled stomach, separ- 

 ated by its own wall from a second spacious cavity lying 

 between that wall and the body wall, and sending a 

 prolongation into each tentacle. The stomach opens 

 freely at its deep end into this second " surrounding " 

 chamber, which is divided by radiating cross walls into 

 smaller partitions, one corresponding to each tentacle. 

 The nourishing results of digestion, and not the food it- 

 self, pass from the stomach into the subdivided or "sep- 

 tate" second chamber. There is thus only one cavity 

 in the animal, separable into a central and a surrounding 

 portion. 



In this respect in having only one body cavity 



sea-anemones and the coral-polyps and the jelly-fishes 



and the tiny freshwater polyp or hydra, and the marine 



compound branching polyps like it agree with one 



6 



