238 MARINE ANIMALS 



Predaceous animals, which hunt for their prey by active movement 

 and find it by means of their sensory organs, do not present any 

 structures peculiar to the pelagial. 



Pelagic and benthic formations compared. — The special adapta- 

 tions required by the pelagial gives its fauna a somewhat different 

 composition from that of the benthal. The groups absent in the pelagial 

 are the sponges and the sessile coelenterates (with the exception of a 

 few pelagic actinians) , the echinoderms, except Pelagothuria and larval 

 stages; and ascidians, brachiopods, and bryozoans. With the exception 

 of the Chaetognatha, worms are few. Lamellibranchs are represented 

 only by Planktomya, and snails only by a few opisthobranchiates and 

 the specialized heteropods and pteropods. Exclusively pelagic are the 

 radiolarians, medusae (with the exception of Lucernaria) , siphono- 

 phores, ctenophores (except Tjalfiella) , chaetognath worms, heteropods 

 and pteropods, and salpas and appendiculates among the tunicates. 

 The copepod crustaceans predominate, composing 90% of the whole 

 fauna, with schizopod and decapod and hyperine forms in addition. 

 Next come the pteropods, siphonophores, and chaetognath worms, 

 while cephalopods and fishes are an important element. 



Distribution of pelagic animals. — The free suspension of the 

 pelagic animals favors their wide distribution, and it is not only active 

 swimmers like the tunny, the bonito, and the golden mackerel, which 

 occur in all the oceans, but also many with poor powers of swimming, 

 such as the deep-sea fish Ceratias couesii, and others. Most of the 

 important genera of sharks are found in both the Atlantic and Indo- 

 Pacific. Many passive forms have the same distribution. The Acantho- 

 metridae (Radiolaria) are astonishingly similar in the warmer parts 

 of the oceans of both hemispheres. 22 The siphonophores of the two 

 oceans are often distinguished only by trivial characters. Pelagic 

 turbellarians are represented in both by the same species. The same is 

 true of most species of the pteropod genera Hyalaea and Cleodora, 

 and of the heteropods Atalanta peroni and Oxy gyrus keraudreni. One 

 may say with Chun that up to the present time no pelagic forms 

 have been discovered in one ocean which are not represented by 

 parallel forms in the other. 23 



The pelagic life of the larvae of many benthic animals is of great 

 importance to their distribution. The length of larval life becomes a 

 governing factor in the extent of the distribution of such forms, vary- 

 ing from 20 to 60 days in the echinoderms to 4 to 7 days in brachiopods, 

 sea anemones, corals, annelids, and snails. Hedley has shown that in 

 Polynesia the gasteropod genera Mitra, Conus, and Cypraea, which 

 have a pelagic trochophore larva, are more widely spread than Melo 



