THE SEA-ANEMONE AND SOME ALLIES 263 



reefs. If coral polyps were to attach to a submerged plateau 

 not too deep for them to live on, the small colony would grow 

 upward and extend itself radially, something like the flaring 

 sides of a shallow wash-basin. As the mass grows larger the 

 polyps at the center would be killed by the detritus collecting 

 from the broken pieces of- coral rock at the wave-beaten 

 margin, and the rock already formed at the center would be 

 worn and scooped out by erosion with coral sand. 



Coral atolls are imperishable bulwarks against the sea, and 

 in some cases have formed the beginning of strips of land 

 sufficiently wide for wild races of men to live upon. 



The Fresh-Water Polyp. There are but tw r o species of 

 fresh- water polyps that are at all common, and these are so 

 small that the casual observer would seldom be aware of their 

 existence, even though they were abundant in his aquarium. 

 The two species are Hy'dra vir'idis (Fig. 131), the green 

 hydra, and Hydra fus'ca, the brown hydra. 



Hydra viridis lives among green fresh-water plants in 

 places where there is abundant sunlight. At rest it holds to 

 a plant with its aboral surface, the remainder of the body 

 floating outward or downward very much like the sea-anemone. 

 The body, which, is cylindrical in form, is about 3 mm. 

 (^ inch) long and .4 mm. (^ inch) in diameter. A little prac- 

 tice will enable a person to distinguish one in an aquarium, 

 for specimens frequently leave the green plants and crawl 

 up the side of the glass nearest the light. They can move 

 either by a slow, creeping movement on the aboral surface, 

 or by the process of " looping," like an inchworm. Hydras 

 exist for a long time in an aquarium, provided there is an 

 abundance of their food-animals, which are usually small 

 Crustacea. The most common of these is a fresh-water spe- 

 cies of Cyclops (resembling Fig. 75), which they benumb 

 with their nettling-threads, and carry to the mouth with their 

 tentacles. 



