ABYSSAL BENTHAL AND PELAGIAL 249 



15.59% 19.34% 58.42% 6.5% 



Depths of 0-1820 m. 1820-3640 m. 3640-5460 m. 5460-7280 m. 



The whole region below, say, 400 m. exhibits a much greater uniformity 

 of environment than the much smaller lighted stratum — absence of sun- 

 light, constant low temperature, regularly decreasing with depth, and 

 almost complete absence of motion, except of extremely slow character. 

 The animal life of this vast area, however, is by no means uniform, 

 but divides into pelagic and benthic in the same way as the lighted 

 stratum. Nevertheless, a number of special characters are common to 

 abyssal animals of both divisions. 



Both number of specimens and number of species regularly decrease 

 with depth, in the benthal as well as in the pelagial; this is shown 

 well in the total collection of the Challenger expedition. 3 More species 

 of animals were secured in the uppermost 180 m. than in the rest of 

 the ocean together (see table, p. 25). The vertical net hauls of the 

 Michael Sars produced 10 fishes and 11 shrimps from 4500 m. to 1500 

 m. (i.e., in a vertical distance of 3000 m.), while from 1350 to 450, 

 in 900 m., 44 fishes and 35 shrimps were secured. The Foraminifera 

 collected by the Gazelle* were as follows: 



The great depths of more than 4500 m. are poor in life, and the most 

 successful dredge-hauls secure only a few specimens. Beebe's 5 direct 

 observations down to 924 m. indicate that there is a greater amount of 

 fish and large invertebrate life at this depth than is demonstrated by 

 net catches. His findings, however, do not alter the conclusions just 



given. 



Animals are frequently not confined to any given depth, in which 

 case they may be referred to as eurybathic as contrasted with steno- 

 bathic forms. Thus of 20 chaetopod worms 6 which descend below 1800 

 m., 12 also occur in the upper 180 m. Eehinocardium australe ranges 

 from to 4900 m.; the bivalve mollusk, Scrobicularia longicallus, 

 ranges from 36 to 4400 m.; the snail, Natica groenlandica, from 4 to 

 2300 m.; and the shrimp, Gennadas parvus, from 600 to 5600 m. 

 Examples of stenobathic forms, aside from purely littoral or purely 

 surface animals, are few. The ascidians, Caesira eugyroides, from 450 

 to 540 m., Pyrosoma elongatum between 750 and 1500 m., and the fish 

 Argyropelecus hemigymnus from 150 to 500 m., may be mentioned. 



