FRESH-WATER ANIMALS 81 



Radiolaria, are almost exclusively marine. Of the 

 next group, or Sponges, some thirty-nine or forty 

 families are recognised, of which one only occurs 

 in fresh water, the rest being exclusively marine. 

 Among the Coelenterata, to which the polypes, sea 

 anemones and corals belong, much the same state of 

 things occurs, for of about seventy families only three 

 have fresh-water representatives ; whilst of the 

 next group, the Echinodermata or starfish, sea- 

 urchins, etc., not a single species inhabits fresh 

 water. In the remaining groups we find the fresh- 

 water forms rather more abundant. The Vermes, 

 or " worms," using the term in its widest sense, 

 have a large number of fresh-water representatives, 

 such as the river worms, Rotifera ; but here also 

 the greater number of families of worms have no 

 fresh-water members. Among the Mollusca we 

 find that several of the best marked groups never 

 get into fresh water, while other groups, as the 

 snails and bivalves, have a large number of fresh- 

 water representatives. The seventh group, or 

 Arthropoda, tells much the same tale ; for though 

 the lower forms, such as the Entomostraca, occur 

 abundantly in fresh water, the higher families are 

 very poorly represented. Turning to the last 

 group, that of the Vertebrates, we find that there 

 are a very large number of fresh-water fish ; yet 

 even here, out of 137 families of fish recognised by 

 Dr. Giinther and other leading authorities, there are 

 only thirty-five that get into fresh water, and many 

 of 'these are only represented by single species. 

 On the other hand the Amphibians, such as newts 



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