38 



MEDUSA. I. 



A. Agassi 7, was of opinion that Ptychogena lac tea was a deep-water species, and he des- 

 cribes, how it is very rapidly killed by the inflnence of the light and the higher temperature of the 

 surface water. In the Massachusetts Bay, however, the conditions are so unfavourable for this species, 

 that its occurrence in this area gives no reliable information with regard to its habits under normal 

 conditions. Bigelow (1914 a, p. 98) states as follows: "The occasional occurrence of Arctic pelagic 

 organisms in Massachusetts Bay and the Bay of Fundy, such as the medusa Ptychogena and the 

 ctenophore Mertensia^ neither of which has been able to establish itself in the Gulf, shows that there 

 are occasional indraughts of the St. Lawrence water into the latter. But ... its influence is either 

 sporadic, or seasonal, not constant." A. Agassiz (1888, p. 128I repeats the statement, that Ptychogena 

 lactea probably is a deep-sea medusa. On the other hand, Vanhoffen (1897, p. 273) mentions a speci- 

 men, found frozen up in the ice in the neighbourhood of Jacobshavn in Greenland, and he remarks 

 (p. 274): "Dieses Vorkommen scheiut mir nicht dafiir zu sprechen, dass diese Art eine Tiefsee-Meduse 

 ist, wie angenommen wurde". Browne (1907, p. 473) likewise remarks: "There is no trustworthy evi- 

 dence that it is a deep-sea form". 



As far as the material, examined by me, is concerned, the depth in which the specimens have 

 been found is only stated in the case of the material, brought home by the "Tjalfe" expedition. The 

 statements of this expedition show that the species may be found in very different depths off the 

 west coast of Greenland. In Umanak Fjord, which is more than 700 m deep, and the deeper water 

 layers of which have a temperature of about 1° C, Ptychogena lactea was found in about 500 m depth. 

 In Disco Bay, the lower strata of which consist, likewise, of cold water, the species was found about 

 350 m below the surface, at a temperature of o°.9 C. On stat. 211, on the Store Hellefiskebanke, it 

 was found about 50 m below the surface; we have no hydrographical data from the station itself, but 

 judging from observations from neighbouring places, made on the same day, the temperature in the 

 depth in question must have been about 1.5—2° C. Godthaab Fjord is one of the fjords, into which 

 the comparatively warm water-masses of the Davis Strait are not admitted on account of a threshold 

 in the mouth of the fjord. On August 30th 1908, when the temperature of the water in the fjord had 

 been under the influence of the conveyance of a whole summers warmth, the temperature was found 

 to be gradually decreasing from 2°.8 at the surface to i°.3 near the bottom (77 m). Ptychogena lactea 

 was, in this fjord, found both near the surface (30 m wire) at a temperature of about 2^.5, and in 50 — 

 70 m depth at about i°.4 C. As the collections of the "Tjalfe" expedition were made with open nets, 

 we may, of course, not be absolutely certain, but that a few animals may have been captured during 

 the hauling up of the net through higher water layers. With regard to the stations 171 (Umanak 

 Fjord) and 125 (Disco Bay) it should be remarked, however, that liauls where also made in higher 

 water layers, and no specimens of Ptychogena were taken in these hauls; it is most probable, there- 

 fore, that the specimens from these stations have actually been captured in the fairly great depths 

 here stated. 



The most important factor determinating the distribution of Ptychogena lactea off the west 

 coast of Greenland seems to be the temperature of the water, the species being only found, where the 

 water is cold. 



As far as the other biological habits of the species are concerned, notliing positively can be 



