2 THE VOYAGE OF H.M.S. CHALLENGER. 



stages of growth without coming into the vicinity of land. These truly pelagic fishes 

 are represented in. the Challenger collection by numerous genera : (species of) Car- 

 charias,^ Xiphias and Histiophorus, Coryphsena, Lirus, Ciihiceps, Psenes, Nomeus, 

 Platystethus, Thynnus, Lepidothynnus, Echeneis, Seriolichthys, Antennarius, Centriscus, 

 Scombresox, Exocoetus, Scopelus (sp.), Diplophos, Astronesthes, Halaphya, {Leptoce- 

 phalus). 



The number of these surface fishes is considerably increased by others which for the 

 greater part of their life inhabit the depth of the ocean, from 100 fathoms downwards. 

 The causes which make these fishes ascend to the surface are not known ; but as some 

 of them have been observed to make their appearance at the surface periodically, we may 

 surmise that this change of habitat is in connexion with their propagation. Indeed, 

 most of them are found at the surface only during the early stages of their growth, Snd 

 it would seem that their ova and fry require for development and growth the higher 

 temperature and the light of the surface water. These fishes connect the surface pelagic 

 fauna with the deep-sea fauna, and are represented in the Challenger collection by the 

 following genera : — Scorpwna (dactyloptera, young), Nealotus (young), Lepidopus 

 (adult and young), Tliyrsites (young), Schedophilus (young), Centrolophus (young), 

 Trachypterus (young), Lophotes (young). Onus sp. (young), Bregmaceros (adult and 

 young), undetermined genera of Pleuronectidse, Scopelus sp. (adult and young), Prymno- 

 thonus. 



The pelagic fauna receives likewise a very considerable contingent from the littoral 

 fauna. A great number of young and undeveloped fishes, which are the offspring of 

 species rarely found in the adult state at any distance from land, occur at the surface in 

 the open sea. Their presence under conditions so widely different from those under 

 which they live when mature, can be explained by the fact that spawn or fry floating on 

 the surface may be driven by currents to great distances from the place where the spawn 

 was originally deposited ; this must frequently happen, especially on oceanic banks 

 which are covered by a small depth of water, and which, therefore, are suitable localities 

 for littoral species. The wide distribution of the same littoral species over large 

 oceanic areas, like that of the Tropical Indo-Pacific, finds thus an easy explanation. 

 The Challenger obtained many immature specimens of such littoral forms in the 

 open sea, as Pimelepterus, Holocentrum, Lichia, Platystethus huttonii, Trigla, 

 Brosmius (?), Onus sp. Fierasfer, Solea (f), Synaptura (f), Hemirhamphus, Belone, 

 Balistes, Tetrodon.^ 



Finally, fully developed specimens of littoral species may also stray or be accidentally 

 driven into the open sea. But these fishes must be considered to be occasional stragglers 



* Very little is known about the propagation of the pelagic species of Carcharias ; all seem to be viviparous, bring- 

 ing forth their young in the open sea, like other viviparous Plagiostomes, or on oceanic banks. 

 2 Whether Branchiostoma should be included in this list is uncertain at present. 



