332 DISCOVERY REPORTS 



intensity of pigmentation corresponds with the bathymetrical distribution of some 

 forms. As Hjort has shown with fishes the differently coloured individuals are 

 distributed according to certain rules within the different layers of water. According 

 to Broch this should also be the case with the Scyphomedusae, and he tried, 

 corroborating the studies of Bigelow, to show that among deep-sea medusae the varia- 

 tion in colour is correlated with bathymetrical distribution— that the more hyaline 

 individuals occur in shallower water, and conversely that those from deeper water have 

 denser pigmentation. 



The Discovery hauls of Periphylla (in a less degree those of Atollo) show, however, 

 that Broch has gone too far in framing a general rule on these lines. As may be seen from 

 the lists of the present series, hyaline and densely pigmented specimens of Periphylla 

 often occur in different layers ; but they are also found in the same hauls near each other, 

 and the various stages of pigmentation are to be found together in the same layers. This 

 is easily understood, as the intensity of pigmentation depends not so much upon the 

 stratum in which the animals live as on the stage of development. Young stages from 

 the surface are generally less darkly pigmented than the older ones living in greater 

 depths. It is in general no doubt true that in Periphylla the size increases with the 

 depth ; because during the development the medusa sinks from shallow into deeper 

 layers, so that the youngest and smallest stages are found near the surface and the adults 

 in the depths; but we find also different developmental stages living in the same strata. 



I cannot, however, agree with Broch that the number of individuals increases with 

 the depth. Broch 's own tables are not sufficient and the numbers too scanty for 

 establishing this observation as a rule or dogma and the present series does not afford 

 any confirmation to his conclusions. 



With regard to the study of pigmentation and bathymetrical distribution it is very 

 much to be regretted that relatively so few hauls were made with closing nets. 

 Whenever such an expression as (-0) or (-50) follows the figure for depth, it is implied 

 that the net (70 cm., 2 m., 4^ m. tow-net, young fish trawl), though fishing for the 

 time indicated at the major depths, was hauled open to the surface or to a higher 

 level. ^ The great majority of the catches have been made in this way ; but the remarkable 

 Phacellophora ornata and one specimen of Pelagia noctihica were caught by the Kelvin 

 tube, having no doubt become entangled with the instrument during its passage to the 

 surface. Thus there is no certainty that all the organisms were really taken at the indicated 

 depth, because some individuals will have been caught during the upward passage of the 

 net, as noted by Bigelow (1909, p. 231). This eminent medusologist has pointed out that 

 such catches are no doubt of great value in obtaining much material, but they are of 

 little use in locating the level from which the specimens were taken unless such records 

 are checked by some other class of observation. One must, however, not fail to recognize 

 the great technical difficulties of closing large towed nets (of 2 m. or 4^ m. diameter), 

 which are the only instruments suitable for collecting Scyphomedusae. Moreover, more 

 specimens with precise indications of depth have been collected by the 'Discovery' 

 1 Discovery Reports. Station-List, 1925-7, p. 5. Cambridge, 1929. 



