392 THE DEEP SEA 



Among the dominant animals, only holothurlans, especially of 

 the order Elasipoda, like Elpidia and Peniagone, are mentioned 

 here. Although other echinoderms among the ophiuroids play an 

 important role, they often seem to have a patchy distribution. 

 Asteroids are also fairly well represented, e\^en by genera with a 

 strongly calcareous skeleton like Eremicaster and Dytaster. Evidence 

 of a curious way in which such animals may obtain part of their 

 calcium supply is illustrated in Fig. 1. 



As mentioned above, tree trunks, branches, and twigs, water- 

 soaked coconut husks, bamboo stems, and a variety of other plant 

 materials are of common occurrence in the deep sea, even astonish- 

 ingly far from land. A thorough examination of the plant debris 

 (Knudsen, 1960) revealed an extraordinary and specialized fauna 

 primarily of boring animals, such as several species of the shipworm- 

 like bivalve Xylophaga. Old tubes are sometimes occupied by small 

 mytilids, amphipods, and worms of the genus Nereis (Kirkegaard, 

 1956, p. 68). 



Quantitatively, the abyssal fauna is poor compared with those of 

 shallower depths. The very few hitherto published data show, 

 however, that locally a considerable abundance of animals may 

 occur even at great depths (Sparck, 1956, p. 200; Zenkevich and 

 Filatova, 1958, p. 156). During recent cruises of several Soviet 

 research vessels a considerable number of quantitative samples 

 have been obtained and summaries of some of the results have been 

 published (Mokijevskij, 1953; Zenkevich and Filatova, 1958; Bel- 

 jaev, 1953 ; Vinogradova, 1956, 1958). 



Distribution 



Not all the animals occurring there can be recognized as true 

 abyssal forms, because they do not spend their whole life in this 

 zone. Examples of this are species of deep-sea eels like Synapho- 

 branchus, which in the early stages of their development lead an 

 epipelagic life typical of Leptocephali lar\'ae. Therefore, the pattern 

 of distribution of such species is mainly influenced by suitable 

 breeding places in tropical or subtropical waters. Another possi- 



