58 THE AQUAVIVARIUM. 



found abundant in the Aquavivarium, and will 

 afford constant opportunity for the use of the 

 microscope. With regard to the other forms of 

 Radiata, few of them have representatives in fresh 

 water. The few that exist are of great interest ; 

 thus the Fresh- water Hydra represents a very large 

 family, found in great abundance in the sea. These 

 creatures are called zoophytes. A family of these, 

 much higher in organization than the Hydra, exists 

 in our fresh waters, and by some writers are referred 

 to the Mollusca. These animals are called Polyzoa.* 

 Although their general structure may be easily 

 recognized by the naked eye, the microscope is 

 required to examine the varied and delicate forms 

 of the polyps of which they are composed. 



Another family of the Radiata found in fresh 

 water are the Sponges. The Fresh- water Sponge 

 (Spongilla Jluviatilis) is not uncommon in the 

 Thames, and the other rivers of England. It would 

 be an elegant addition to the fresh- water vivary, 

 but I have never been able to make it live, 



* A monograph of the British Fresh-water Polyzoa, by 

 Professor Allman, of Edinburgh, is announced for publica- 

 tion by the Kay Society. 



