

268 MARINE ANIMALS 



abyssal, while 28 of the 91 genera reach depths below 800 m. The 

 number of deep-sea fishes enumerated by Brauer 36 is 309 genera with 

 1007 species, of which 131 genera and 397 species are pelagic. In the 

 groups represented, this fauna differs conspicuously from that of the 

 lighted zones; few spiny-rayed fishes reach the depths, and lopho- 

 branchiates and plectognathids are wanting. 



Archaic forms in abyssal waters. — The necessity for special 

 adaptation for life in the deep sea, of which only a certain proportion 

 of animals are capable, produces the relative poverty of the deep-sea 

 fauna. While the struggle with the physical environmental forces is 

 increased, the competition between species is diminished. This is con- 

 sequently one of the places where ancient forms have been able to 

 maintain themselves. Such archaic forms are not absent from the 

 lighted zone — as illustrated by the horseshoe crab, the brachiopod 

 Lingula, and amphioxus — but they are relatively more abundant 

 in the abyssal depths. Many of the deep-sea stony corals are closely 

 related to Mesozoic and early Tertiary forms. The sea-urchin families 

 Salenidae, Echinothuridae, and Ananchytidae, which reach their maxi- 

 mum development in the Cretaceous, were thought to be extinct until 

 the dredge brought living representatives to light from the deep sea. 

 Of the more recent Clypeastridae, which originated in the Cretaceous, 

 only the two oldest genera, Echinocyamus and Fibularia, go deeper 

 than 400 m. A. Agassiz 37 remarks that the forms with greatest range 

 in depth are also those with greatest span in time; the living littoral 

 forms extend only into the late Tertiary. The stalked crinoids, which 

 were numerous and widespread in the earlier geological periods, are 

 now confined to the deep sea. The cephalopods Spirula and Nautilus 

 belong to the deep sea, though Nautilus may rise to the surface. Among 

 abyssal decapod crabs the representatives of the otherwise Triassic 

 family Eryonidae {Willemoesia, for example) are notable, as is the 

 great number of such primitive groups as the Peneidae and Caridae. 

 Among fishes the chimaeras and the shark, Chlamydoselachus, related 

 to the Devonian Cladodus, may be named as archaic. Among the bony 

 fishes, forms with soft rays predominate, as they do in fresh water. The 

 more recent spiny-rayed fishes have as yet scarcely found their way 

 into the depths. 



Although there is this easily recognized archaic element of the 

 abyssal fauna, the ancient character does not apply to the whole, as 

 it does for example to the land fauna in Australia. The deep sea is not 

 isolated, and the way into the depths is continuously available to ani- 

 mals of the lighted zone. The genus of crabs Ethusa, in which the same 

 species exhibits reduction of eyes with depth, has been regarded as an 



