600 ZOOLOGY SECT. 



fish (Regalec*its), attain a great size, and are toothless. When 

 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 larvae 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, Chsetopods, etc. ; 

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



The Fresh-water Fauna presents certain characteristic 

 features, and is divisible into fluwatile 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 the whole of the Phylactolaemata, and one or 

 two genera of Gynmolaemata, are fresh-water forms ; so also are 

 many of the Oligochaeta, e.g., Nais and Tubifex, but none of the 

 Polychaeta. Fresh-water Entomostraca are numerous and abund- 

 ant, and belong to all orders except Cirripedia ; among Malacos- 

 traca there are only some Amphipods and Isopods, 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 lacus- 

 trine 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 

 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 



