36 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



waters. The host of sponges is represented only by the small family 

 Spongillidae. Of the coelenterates with their wealth of forms one 

 encounters only a few hydroids with world-wide distribution, such as 

 Hydra and Cordylophora (the latter more commonly in brackish 

 water up to 1.3% salinity), and a very few fresh-water medusae in 

 widely scattered localities. The flatworms are relatively well repre- 

 sented, with rhabdocoel and triclad turbellarians, but the nemertines 

 are very few. Rotifers are present in greater numbers than in the sea, 

 and the Gastrotricha are confined to fresh waters. The fresh-water 

 annelids include a few leeches, the somewhat uniform group of 

 limnicolous oligochaetes (aquatic relatives of the earthworm), and a 

 few quite isolated species of polychaetes. Bryozoa are well represented 

 in a single family of Entoprocta, otherwise very rare. Crustaceans, 

 especially Entomostraca, form an important element in the primary 

 fresh-water fauna. Despite the presence of these primarily aquatic 

 animals, the secondarily aquatic animals, such as insects, arachnids, 

 and pulmonate snails, dominate the life of fresh water. 



Marine invasions of fresh water. — The immigration of marine 

 forms into fresh water has taken place for ages and still continues. 

 Fresh-water forms whose close relationship to marine forms indicates 

 relatively recent entrance into the new habitat are naturally limited 

 in their distribution. Older groups have gained wider distributions. 

 These more or less universal fresh-water animals are usually sharply 

 defined groups, well separated from their marine relatives. Among 

 such groups, with a long independent phylogenetic history, are the 

 fresh-water sponges, the fresh-water bryozoa, and the Ostariophysea 

 among the bony fishes (characins, gymnotids, cyprinids, and silurids). 

 In the case of immigration which is recent or still continuing, fresh- 

 water groups are related to the inhabitants of the neighboring seas 

 in various degrees. The species may occur in the ocean also; the fresh- 

 water species may belong to genera otherwise marine; or fresh-water 

 genera of families otherwise marine may occur in limited areas. Such 

 relations all indicate recent immigration, ecologically speaking. The 

 new inhabitants of fresh water have not had time to diverge greatly 

 from their marine relatives. 



Numerous examples of regional fresh-water forms are found among 

 the mollusks, crustaceans, and fishes. The marine genus of snails 

 Tectura has a single fresh-water species, Tectura fluminalis, in the 

 Irrawaddy River. The marine genus Cerithium is replaced in the fresh 

 waters of India by Brotia; Nassa by Canidia. 11 Among mussels, the 

 rock-boring Pholas and other marine forms occur in Trinidad 18 km. 

 from the sea, in water entirely fresh, though still subject to tidal move- 



