94 DISCOVERY REPORTS 



The Leptolina, comprising the Antho-, Lepto- and Limnomedusae, are presumably all meropelagic, 

 and the majority of them are neritic, being derived from hydroids which are attached to objects on 

 the bottom in the coastal regions. In many species the duration of the pelagic life is so short that the 

 medusae cannot possibly be carried far away from the coastal waters by the currents, but in some 

 species the pelagic phase is long enough to allow them to drift into remote regions far from their 

 place of origin, either along the coast, if the current takes that direction, or out into the 

 open sea. 



Some few species of Leptolina, however, must be designated as truly oceanic, in so far as they are 

 regularly found far out in the great oceans. Some of them may have littoral hydroids, but an oceanic 

 existence is made possible by the ability of the medusa to propagate by budding, producing several 

 successive generations of medusae. In other instances we know nothing about their propagation, 

 either sexual or asexual ; we can only state that they do occur in the open oceans, evidently independent 

 of the coasts. 



The Trachylina, comprising the Trachy- and Narcomedusae, are holopelagic, passing their whole 

 life-cycle in a free-swimming condition. Some of them are decidedly bathypelagic, occurring in the 

 deep layers only; others are epipelagic, their occurrence being restricted to the surface waters, down 

 to depths of lOo to 250 m. Again, some species are bathypelagic in tropical and subtropical regions, 

 whereas they may be found nearer the surface in cold areas, indicating that they are more dependent 

 on temperature than on the actual depth of the waters. There are also some species which have their 

 principal habitat in the upper layers, but which occasionally, or under certain conditions, descend 

 to quite considerable depths. In some cases, therefore, the distinction between epipelagic and 

 bathypelagic species is to some degree arbitrary. 



Distribution of the Species of Hydromedusae taken by the Discovery 



Investigations 

 Tables 2-4 give the geographical distribution of the Hydromedusae collected by the Discovery 

 Investigations during the years 1926 to 1937, but in order to complete the picture, records of these 

 species from previous expeditions have also been included. The species have been arranged in the 

 following groups : 



(i) Neritic species of Leptolina (Table 2, p. 124). 



(2) Oceanic species of Leptolina and Trachylina, predominantly epipelagic (Table 3, p. 125). 



(3) Oceanic species of Leptolina and Trachylina predominantly bathypelagic (Table 4, p. 126). 

 The seven new species, the further distribution of which is not yet known, are not included in the 



tables. In each table the species are arranged approximately according to their distribution from South 

 to North. This gives a general impression within each group of the penetration of the species away 

 from or into the Antarctic waters, and also indicates which of the species are bipolar. 



The regions shown in Tables 2 and 3 and used in the discussion of the distribution of the Hydro- 

 medusae are based on the proposals put forward by Ekman in his Zoogeography of the Sea (1953). 

 I have made some slight alterations in the names of the regions, for example West- Atlantic for East- 

 American, East- Atlantic-tropical for West- African-tropical etc., and I have separated the East- 

 African-tropical area from the comprehensive Indo-West-Pacific-tropical, region as a distinct region. 

 Each of these large regions may of course be subdivided into minor faunistic provinces, but for the 

 sake of simplification I have refrained from doing this as it would make the discussion unnecessarily 

 complicated. The Antarctic region is, however, divided into three sectors: the Pacific, Atlantic and 

 Indian sectors, because the material available varies very greatly in these three areas. 



