INTRODUCTION 



15 



Porifera 

 or Sponges 



Coelenterata 



Echinodermata 



Platyhelminthes 

 or Flatworms 



TURBELLAEIA 



OR Feee-living 

 Flatworms 



Trematoda 

 OR Flukes 



Cestoda 

 OR Tapeworms 



Nemertina 



Nemathelminthes 

 or Round- 

 worms 



Preeminently marine; fresh-water bodies shelter 

 a considerable number of characteristic siliceous 

 forms all embraced in a single family, SpongilUdae. 



These are described in Chapter X, page 301. 

 A group manifesting great variety and abundance 

 in the sea, represented in fresh water by a very few 

 widely scattered types, both polyps and medusae, 

 all belonging to one class, the Hydrozoa; other 

 classes confined to the sea. 



For Hydrozoa see Chapter XI, page 316. 

 Includes crinoids, brittle-stars, starfish, sea-ur- 

 chins, and sea-cucumbers; not represented in fresh 

 water by a single known species. 

 Four classes are recognized, all of which furnish 

 important representatives to the fresh-water 

 fauna. 



Common in salt and fresh waters; species found 

 in the latter generally insignificant in size. A few 

 are terrestrial in moist environments. 



See Chapter XII, page 323. 

 All species parasitic; many in or on fresh-water 

 animals; with developmental stages, embryos 

 (miracidia) and larvae (cercariae) that occur free- 

 swimming in fresh water. 



See Chapter XIII, page 369. 

 Exclusively parasitic forms. Adults common in 

 fresh-water vertebrates; developmental stages in 

 various aquatic animals, mostly invertebrates; 

 rarely with a free-swimming embryonic stage. 



See Chapter XIII, page 424. 

 Mostly marine; a very few species of small size 

 and simple organization widely distributed in 

 fresh water. 



See Chapter XIV, page 454. 

 A confused group of three classes showing little 

 similarity in structure and associated in a single 

 phylum largely as a matter of convenience. All 

 are well represented in the fresh-water fauna. 



