16 FRESH-WATER BIOLOGY 



NEMATODA Both free and parasitic forms common in all sorts 



OR TRUE ROUND- o f environments; free-living species most abun- 

 dant in fresh waters and in moist soils; parasitic 

 species common in fresh-water hosts. 

 For free- living Nematoda, see Chapter XV, 

 page 459- 



For parasitic Nematoda, see Chapter XVI, 

 page 510. 



GORDIACEA Parasitic in young life in insects: adult stage free- 



OR HAIR SNAKES ^y^g m f resn wa ter. 



See Chapter XVI, page 535. 



ACANTHOCEPHALA Exclusively parasitic, without trace of alimentary 

 OR THORNY- system. In many fresh-water hosts. Adults in 



HEADED WORMS . , i i r f ^ i 



vertebrates; larval forms imperfectly known, 

 parasitize invertebrates. 



See Chapter XVI, page 542. 

 Trochelminthes Among the most characteristic of aquatic or- 



orTrochal ganisms. Favorite objects of study with the 



Worms 



early microscopists. 



ROTATORIA Microscopic free-living forms, very rarely para- 



OR WHEEL s i t j c> Abundant in fresh-water bodies of all sorts: 

 ANIMALCULES . ,, 



rare in the sea. 



See Chapter XVII, page 553. 



GASTROTRICHA Minute free-living forms. Abundant in fresh water 

 to which the group is limited. Imperfectly known. 



See Chapter XVIII, page 621. 

 Coelhelminthes Two classes in fresh water both well represented; 



(Annelida) other classes exclusively marine, 



or Segmented 

 Worms 



CHAETOPODA OR One sub-class (Polychaeta) confined to the sea 

 BRISTLE WORMS save f or rare types in fresh-water bodies near the 

 ocean; the other sub-class (Oligochaeta) found 

 mostly in fresh water and on land. 



See Chapter XIX, page 632. 



HIRUDINEA Both free-living and parasitic species, the former 



OR LEECHES mostly in fresh water with a few species also on 

 land in moist regions; rarely marine, as ectopara- 

 sites of fishes. 

 See Chapter XX, page 646. 



