636 ZOOLOGY sect. 



brought to the surface, the expansion of the gases in the interior 

 of the deep-sea Teleosts often bursts the air-bladder, and 

 produces a general disintegration of the tissues. 



Plankton, Nekton, and Benthos. — Besides being arranged 

 with regard to their relations to the shore, the surface of the 

 ocean, and its bed, marine animals are also conveniently classified 

 on the basis of their capacity for movement. Many forms, such 

 as Medusae, Siphonophora, Ctenophora, Salps, and numerous 

 pelagic larvse are carried along passively by oceanic currents, their 

 own powers of progression being of the feeblest. Such animals 

 together constitute the Plankton, or "drifting-fauna." Others 

 swim actively by means of fins or other appendages, such as the 

 pelagic Teleosts and Elasmobranchs, Schizopods, Prawns, and 

 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, Chastopods, fee; 

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



The Fresh-water Fauna presents certain characteristic 

 features, and is divisible into fluviatih forms, inhabiting streams 

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

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

 few Foranfiinifera and no Radiolaria. Among Sponges there is 

 only a single fresh-water family, the Spongillidas : 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 

 Phylactolfemata, and one or two genera of Gynmolaemata, are 

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

 and Tubifex, but very few Polychseta. 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 larva3 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 Teleostei, the 

 Siluroids and Salmonidffi 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 



