TESCH— PTEROPODA AND HETEROPODA 187 



3. DISTRIBUTION. 



Horizontal distribution. As has been repeatedly pointed out, the Heteropods are 

 typical inhabitants of tropical and subtropical waters. Like many other pelagic organisms 

 they seem to extend within these regions over vast areas and some species are almost 

 universally distributed. The occurrence of Protatlanta souleyeti in the Indian Ocean may 

 first be mentioned; this species had been hitherto only observed in the Atlantic*. All 

 other Atlantidse here recorded were already known to inhabit the area explored. Among 

 the Carinariidpe Pterosoma, which was found off the coasts of New Guinea, in the Channel 

 of Formosa, between the Admiralty Islands and Japan, near Sydney, and in the gulf 

 of Bengal, has been obtained by the Sealark from the western part of the Indian 

 Ocean ; further the three species of Cardiapoda of the Siboga Expedition were caught 

 in this same region. As to the Pterotracheida? the most notewoi'thy fact is the catch of 

 Pterotrachea (s. s.) scutata, a species known to inhabit the Mediterranean (Gegenbaur, 

 Oberwimmer) and also the eastern part of the Atlantic (Vayssiere under the name Firola 

 gegenbauri). Pterotrachea (Euryops) mutabilis has been recorded recently by the Siboga 

 Expedition from the Malay Archipelago. Finally Firoloida koivalewskyi, which not only 

 inhabits the seas around the Canaries and the Malay Archipelago, seems to be abundant 

 in the tropical Indian Ocean. 



It is remarkable that most of the Heteropods were found during the second part of 

 the expedition, and especially in the Amirante Group. To what cause this must be 

 ascribed, is difficult to say. The hydrographical conditions of the area do not seem to 

 differ essentially from other parts of the Indian Ocean visited by the Sealark. Whether 

 perhaps we may explain the fact by some particular method of pelagic fishing we shall 

 now try to find out. 



Vertical distribution. So far as I know, Oberwimmer was the first to impose upon 

 the Heteropods a similar rule as had been erected by many naturalists already for the 

 Pteropods, viz. migrations to the surface as soon as darkness begins and disappearance at 

 daybreak t. According to this author both groups of organisms were most plentiful at the 

 surface from 6.45 p.m. to 8.45 p.m., afterwards became rarer, and showed from 3.45 a.m. to 

 5.30 A.M. another though smaller maximum, whereas surface-fishing during the day gave 

 only poor results. 



From the foregoing list of the stations from which Heteropods have been recorded it 

 may be inferred that the following species were caught at or near the surface down to 

 25 fms. 



Protatlanta souleyeti, Stat. n. 



Atlanta peroni, Stat. P, u, hh. 



Atlanta lesueuri, Stat, n, u. 



Atlanta injiata, Stat. n. 



* See my recent publication : Monograph of the Atlantic!^, with description of the species in the 

 Leyden Museum, Notes from the Leyden Museum, vol. xxx. p. 29, 1908. 



t Heteropoden und Pteropoden, Sinusigera, gesammelt von S.M. Schiff " Pola," 1890 — 1894, Den/csclir. 

 Kais. Ah. Wiss. WUn, Mrtthrm.-Naturwiss. CI., Band 65, 1898, pp. 574, 575. 



24—2 



