616 ZOOLOGY SECT. 



Squids they form the Nekton, or " swimming-fauna." Others, 

 again, have no natatory organs, and are either permanently fixed, 

 like Zoophytes and Stalked Crinoids; or move by creeping over 

 the sea-bottom, like Starfishes, Holothurians, Chaetopods, &c.,; 

 such forms constitute the Benthos, or " bottom-fauna." 



The Fresh-water Fauna presents certain characteristic 

 features, and is divisible into fiuviatile forms, inhabiting streams 

 and rivers, and lacustrine forms, inhabiting lakes. It is very rich 

 in Lobosa, Heliozoa, Flagellata, and Infusoria, but contains very 

 few Foraminifera and no Radiolaria. Among Sponges there is 

 only a single fresh-water family, the Spongillidae : among Hydrozoa 

 only four genera, Hydra, Cordylophora, Limnocodium, and Limno- 

 cnida ; and among Actinozoa and Ctenophora not a single species. 

 There are also no fresh-water Echinoderms or Brachiopods, but 

 many Turbellaria, a few Nemertinea, and numerous Nematoda. 

 Among Polyzoa one genus of Endoprocta, the whole of the 

 Phylactolsemata, and one or two genera of Gymnolaemata are 

 fresh- water forms ; so also are many of the Oligocha3ta, e.g., Nais 

 and Tubifex, but very few Polychaeta. Fresh-water Entomostraca 

 are numerous and abundant, and belong to all orders except 

 Cirripedia ; among Malacostraca there are only some Amphipods 

 and Isopods, Anaspides and its allies, and Fresh-water Shrimps, 

 the various genera of Fresh- water Crayfishes, and a few Crabs. 

 The larvae of many Insects are aquatic, and there are several 

 aquatic Spiders. Pelecypods and Gastropods furnish abundant 

 fluviatile and lacustrine forms, although belonging to comparatively 

 few genera ; Cephalopods, on the other hand, are wholly absent 

 from fresh waters, as also are the Tunicata. Among Fishes there 

 are several species of Lampreys, and numerous Teleosts, the 

 Siluroids and Salmonidae being especially characteristic. There 

 are no fresh-water Elasmobranchs, with the exception of one or 

 two genera of Sting-Rays in the rivers of tropical America ; but 

 the Ganoids are a characteristic fresh-water group, although some 

 forms, such as the Sturgeons, migrate to the sea at certain seasons. 

 The Dipnoi are exclusively fluviatile, or live in swamps caused by 

 river overflow, and the perennibranchiate Amphibia, as well as the 

 larvae of the caducibranchiate forms, are characteristic members of 

 the fresh-water fauna. Many Chelonians and Crocodiles, such 

 Birds as Ducks and Grebes, and such Mammals as Otters, the 

 Hippopotamus, and Ornithorhynchus, may also be included in the 

 fresh-water fauna, and some Dolphins are purely fluviatile. 



The animal inhabitants of large lakes, like those of the sea, are 

 divisible into littoral, pelagic, and deep-water forms : the pelagic 

 forms are, in this case also, characterised by their extreme 

 transparency. Mention must also be made of animals dwelling in 

 deep subterranean caves, shut off from sunlight, such as Proteus, 



