328 THE DEEP SEA 



deposits. The family Brisingidae (Asteroidea) is the most primitive 

 form of the order Forcipulata. It is abundantly represented in the 

 ocean depths. Seven of the seventeen genera in this family are 

 represented by forty-one species in abyssal depths. 



Phanerozonia, the most primitive order known from the Mid- 

 Cambrian, also has some representatives. The Porcellanasteridae 

 is the most primitive family in this order. Furthermore, forty-seven 

 species of this family live only in the ocean depths. Fifteen species 

 of Echiuroidea, a very old and primitive group of worms, are known 

 to inhabit the abyssal depths only. To these could be added a great 

 number of Crustacea and Polychaeta. There are 151 species, even 

 without the latter, i.e., at least 15% of all species taken from 

 depths exceeding 2000 m. If we consider only forms related to 

 fossils from the Paleozoic, they alone contribute 96 species, i.e., 

 about 8% of all known deep species. In contrast, only twelve old 

 and primitive marine species are cited by Menzies and Imbrie 

 from shallow water (even with the addition of another 4 species 

 of Nautilus) and these form only 0.005% of the total in shallow 

 seas, i.e., they are only one-thousandth of the total. Except for the 

 Foraminifera, the groups these authors are chiefly concerned with 

 (Bryozoa, Brachiopoda, Scleractinia) are definitely not charac- 

 teristic of the abyssal fauna. 



Doubtless the abyssal fauna has young representatives (geo- 

 logically speaking) and lone representatives. Similarly, the shallow- 

 water fauna contains archaic, primitive forms. However, this fact 

 does not preclude our basic position. Thus, the abyssal fauna is 

 distinguished by its obvious slow evolutionary development, 

 which took place in a very homogeneous environment. This is 

 shown by the exceedingly high dispersal rate of its inhabitants. 

 Not only the abyssal fauna, but also the cave-dwelling fauna (to a 

 less extent), the soil-inhabiting fauna and the interstitial fauna of 

 the beaches along the seashore lag behind this evolutionary 

 development. The supposition is that a rapid formation of the 

 abyssal fauna is in conflict with a slow formation in waters lacking 

 such a fauna throughout its geological history. The Japan Sea, for 

 example, has none. The peculiar abyssal fauna in the deep de- 

 pression of the Polar Basin is still in its infancy. We believe that 



