THE PELAGIAL 241 



between day and night. At night numerous deep-sea forms come to 

 the surface, even from great depths, to return before daybreak. These 

 are followed by predatory forms which live on them. The schools of 

 herring come to the surface at night, remaining at considerable depths 

 during the day. 



Important and readily understandable differences exist between 

 the pelagial of the deep sea and that of shallow seas; one may dis- 

 tinguish between these as oceanic and neritic pelagial. The oceanic 

 pelagic life is made up of animals independent of the bottom through- 

 out their development; these animals are called holopelagic or holo- 

 planktonic. To this group belong all radiolarians, many Foraminifera, 

 the medusae without a sessile stage (many Hydromedusae but few 

 Scyphomedusae, among the latter Pelagia noctiluca), most siphono- 

 phores and ctenophores, the chaetognath worms, copepods and 

 hyperines among crustaceans, heteropods and pteropods, appendicu- 

 lates, salpas, and pyrosomas, some sharks, a great number of bony 

 fishes with air bladders, and some without, and finally the Cetacea. 



The neritic pelagial includes, in addition to the holopelagic forms, 

 animals which are dependent on the bottom during part of their 

 existence; to these the terms hemipelagic or meroplanktonic are ap- 

 plied. To this group belong all the medusae with alternating genera- 

 tions, the larvae of numberless benthonic animals, and the floating 

 eggs of many fishes. The water fleas of the genera Podon and Evadne, 

 whose eggs sink to the bottom, and the ostracod Philomedes brenda, 

 which comes to the surface in May to breed but otherwise lives on 

 the bottom, are hemipelagic. It includes many cephalopods (Sepiola, 

 for example) , most sharks and rays, and many bony fishes, even some 

 active swimmers like Julis and Coris, which rest on the bottom at 

 night. Finally, the neritic pelagial includes the sea turtles and seals, 

 and especially the walrus, which feeds on benthic animals. The time 

 as well as the season of dependence on the bottom vary with the 

 different animals, so that periodic changes in the composition of the 

 neritic pelagial are much more pronounced than in the oceanic. 



The neritic pelagial is bounded in general by the 200-m. contour, 

 and thus includes the banks as well as coastal waters. This boundary 

 is the obvious result of dependence on the bottom. The inhabitants of 

 the neritic and oceanic pelagial are mixed at the boundary, and cur- 

 rents carry the neritic forms out and the oceanic ones in, generally 

 to their destruction, although the eels are a marked exception. To what 

 extent the holopelagic animals are destroyed by modified physical 

 conditions, such as greater turbidity of the water, in the neritic area, 

 and to what extent they are destroyed by the competition of 



