BATHYMETRIC EANGE. 231 



400 fathoms). It is very desirable that more hauls be made with closing 

 nets adapted to the capture of large organisms, many of those now in use 

 being, as Apstein (: 06, p. 277) points out, unfitted for this purpose. 



Next in value to closing-net hauls are serial hauls, based on the compara- 

 tive method, with open tow-nets ; that is, hauls from different depths made 

 at one station at the same time or in immediate succession with nets of 

 the same diameter and size of mesh. This method, owing to the amount of 

 time necessary to carry it out, has seldom been used comprehensively. Far 

 the most important instance, in the present connection, is afforded by the 

 investigations of Murray ('99) in the North Atlantic. Though none of the 

 typical intermediate Medusae were taken during this Expedition, Aglantha 

 dicjltale is recorded (Guenther, : 03, p. 427) as reaching its greatest abun- 

 dance at the very great depth of over 1,000 fathoms. But the fragmentary 

 condition of all the specimens {minus manubrium, gonads, tentacles), to- 

 gether with the season of the year (November) when the hauls were made, 

 strongly suggests that the captures are not to be taken as indicating the 

 normal occurrence of this surface form at such a depth, but rather that the 

 net encountered swarms of dead specimens in the process of sinking to 

 the bottom after the discharge of their sexual products. 



The evidence afforded by captures made in ordinary open tow-nets from 

 considerable depths is much more extensive, but must be analyzed with even 

 greater discrimination than that of the closing nets. Especially must we 

 guard against any tendency to locate specimens from the lowest point of 

 vertical hauls, as has often been done in the past. Such a method is of 

 course entirely misleading ; the actual evidence given by any one haul is no 

 more precise than that its contents came from somewhere between the lowest 

 point attained and the surface. Obviously the shorter the column of water 

 through which the net is towed the more accurately is the zone of origin of 

 its contents located. Conversely, when vertical hauls are made from near 

 the bottom to the surface they indicate merely that the contents came from 

 somewhere between the bottom and the top. Such hauls are often of great 

 value in obtaining material ; they are of little (unless used in conjunction 

 with the comparative method already mentioned) in locating the level from 

 which the contained specimens were taken. And of course even less instruc- 

 tive, from this point of view, are sporadic records of captures of pelagic 

 animals in the trawl or dredge, unless such records are checked by some 

 other class of observations. The importance of this precaution seems to 



