INTRODUCTION 



17 



Arthropoda 



Crustacea 



ARACHNroA 



Insecta 



Tentaculata 



Bryozoa 

 OR Moss 

 Animalcules 



Three of the five classes usually recognized are 

 found in fresh water. 



Only one sub-class, Cirripedia or Barnacles fur- 

 nishes no fresh-water representatives. The others 

 are well represented in the fresh-water fauna. 

 With few exceptions free-living forms. 

 For Phyllopoda see Chapter XXI, page 661. 

 For Cladocera see Chapter XXII, page 676. 

 For Copepoda see Chapter XXIII, page 741. 

 For Ostracoda see Chapter XXIV, page 790. 

 For Malacostraca see Chapter XXV, page 828. 

 Chiefly terrestrial with some parasitic forms. 

 One or two spiders have secondarily invaded fresh 

 water. ^ Among the mites one sub-order, the Hy- 

 dracarina, is exclusively aquatic. Nearly all 

 species inhabit fresh water. 



For Hydracarina, or Water Mites, see Chapter 

 XXVI, page 851. 

 Two aberrant groups often attached to this 

 class are the following: 



Linguatulida, exclusively parasitic, occur rarely 

 in fresh-water hosts. 



Tardigrada, minute free-living forms known as 

 water bears; a few species not uncommon in 

 fresh water. 

 Typically land animals which in some cases 

 (especially for developmental stages) have gone 

 into fresh water and manifest secondary adapta- 

 tions to aquatic life. 



See Chapter XXVII, page 876. 

 Of two classes, one, the Brachiopoda, is exclusively 

 marine. The other follows: 

 Sessile animals, nearly always colonial; exclu- 

 sively free-living; chiefly marine but with some 

 fresh-water forms widely distributed. 

 See Chapter XXVIII, page 947. 



