Aquatic Organisms 163 



METAZOANS 



Hydras are the only common fresh-water representa- 

 tives of the great group of Ccelenterates, so abundant 

 in the seas; and of hydras there are but a few species. 

 Two of these, the common green and brown ones, 

 H. virdis and H. fusca, are well enough known, being 

 among the staples of every biological laboratory. 

 Pedagogically it is a matter of great good fortune that 

 this little creature lives on, a common denizen of fresh- 

 water pools; for its two-layered sac-like body repre- 

 sents well the simplest existing type of metazoan 

 structure. 



Hydras are ordinarily sessile, being attached by a 

 disc-like foot to some solid support or to the surface 

 film, from which they often hang suspended. But at 

 times of abundance (and under conditions that are not 

 at present well understood) they become detached and 

 drift about in the water. A hydra of a brick-red color 

 swarms about the outlet of Little Clear Pond at Saranac 

 Inn, N. Y., in early summer, and drifts down the out- 

 flowing stream, often in such abundance that the water 

 is tinged with red. The young trout in hatching ponds 

 through which this stream flows, neglect their regular 

 ration of ground liver, and feed exclusively upon the 

 hydras, so long as the abundance continues. The 

 hydras play fast-and-loose in the stream, attaching 

 themselves when they meet with some solid support, 

 and then loosening and drifting again. 



Clear, sunlit pools are the favorite haunts of hydras, 

 and the early summer appears to be the time of their 

 maximum abundance. They attach themselves mainly 

 to submerged stems and leaves, and to the underside of 

 floating duckmeat. They feed upon lesser animals 

 which abound in the plancton, and, multiplying rapidly 

 by a simple vegetative process of budding with subse- 



