GERMAN PLANKTON INVESTIGATIONS. 335 



ship while still in motion, with sufficient rope to 

 allow it to remain under but near to the surface. 

 It is allowed to remain out for ten minutes or a little 

 longer, then hauled in and the plankton emptied into a 

 glass trough. If the net is allowed to remain out too long, 

 it becomes so blocked up with plankton that it filters 

 badly. In this way the pressure on the net becomes con- 

 siderably increased, and it is liable to become ruptured. 

 When used during the above-mentioned time the Appen- 

 dicularia are captured uninjured and alive. If the catch 

 is very slimy, the net gets stopped up sooner than usual. 

 Although Hensen described the construction of this net, 

 and its mode of use, fourteen years ago, nets of similar 

 character have been described by other investigators. 

 Borgert has described a modification which differs from 

 the Hensen " Korbnetz " in that it is quite free behind. 

 Hensen considers that the strain on the net in that case 

 would be too great, and that the net would be easily torn, 

 but Borgert has not found such to be the case. The 

 calculation of the volume of water which passes through 

 this net in a particular instance is complicated, and 

 depends on a number of variable factors, among which are 

 the area of the opening, the length of time the net is out, 

 the speed of the steamer, the angles formed by the rope 

 attached to the net, to the perpendicular, when being let 

 out and when being pulled along. This net serves 

 admirably for the capture of plankton from a steamer 

 when going at full speed. 



The latest form of quantitative plankton net, and one 

 which, like the Korbnetz, is capable of being used from 

 a steamer going at full speed, is one devised by Petersen 

 and improved by Hensen, here designated as the Petersen- 

 Hensen net (fig. 11). This net combines a strong pro- 

 tective covering, with an aperture of the smallest possible 



