THE BEXTHAL 179 



ference between tide marks amounts to about 2 in. In the Mediter- 

 ranean it is only 35 cm. on the average. Where funnel-shaped bays 

 confine the tide wave, the tidal differences are much greater; thus at 

 Chepstow in the Bristol Channel, and in the Strait of Magellan, 

 11.6 m.; at Granville on the west coast of Normandy, 12.4 m.; in the 

 Bay of Fundy, Nova Scotia, up to 15.4 m. The upper limit of the quiet 

 water is also variable with local conditions. 



A supratidal spray zone may be distinguished above high tide mark ; 

 its inhabitants are a mixture of terrestrial and marine animals, and 

 are exposed to a special set of conditions. In many regions, animals 

 from just below the low tide mark differ significantly from those of 

 the deeper, but still wave-affected, waters. Such a region is spoken of 

 as adtidal. 2 



The littoral benthos. — The littoral bottom fauna is the common 

 mother of all water-breathing aquatic faunae, fresh water as well as 

 marine. The faunae of the deep sea as well as the life of the open ocean 

 are nothing more than specializations which are derived from the lit- 

 toral bottom faunae by special adaptations. Bottom life at moderate 

 depths in coastal waters requires the fewest adaptations to physical 

 conditions, so that all groups of marine animals are represented, while 

 many groups are wholly wanting in the pelagial and others are repre- 

 sented only by a few aberrant forms, as in echinoderms and mollusks. 

 Pelagic animals may always be traced to an origin from bottom- 

 dwelling forms, and differ from them regularly by their swimming 

 organs and arrangements to secure suspension. This is especially evi- 

 dent in the pelagic medusae, with their fixed ancestral stage; only 

 highly specialized forms of worms occur in the open water, and the 

 pelagic mollusks, heteropods and pteropods, are plainly derived forms. 

 Bottom animals, on the other hand, could not enter the abyssal depths 

 until after the pelagic animals and plants established a food supply 

 for them. 



Marine animal life is most richly developed both in number of 

 species and individuals in the littoral. The dredge nets of the Challenger 

 rarely produced more than 10 to 15 individuals of a species from 

 depths over 1800 m.; in about 900 m., hundreds of specimens of single 

 species, holothurians and pyenogonids for example, were taken; and 

 at about 180 m. on the continental coast vast numbers of single species 

 were dredged. 3 The lists of species show almost equally as great dif- 

 ferences according to depth as the numbers of specimens. The striking 

 differentiation into numerous species in the littoral fauna is favored 

 by the great local variation in conditions and by the frequent opportu- 

 nities for isolation. Island regions, with their extreme elaboration of 



