624 DEPTHS OF THE OCEAN chap. 



between 500 and 750 metres in the northern section, whereas 

 in the south they were seldom captured by the net towed at 500 

 metres, though present in large numbers at a depth of 1000 

 metres. 



The results of these investigations clearly show that the 

 dark-coloured fish, the deep-red prawns, and other organisms 

 are limited to the deep parts of the ocean beyond 500 metres. 

 This bathypelagic region may, however, be subdivided into 

 various layers. We thus recognise a layer varying according 

 to geographical position between 500 and 800 metres contain- 

 ing the light-coloured species of Cyclothone and the bright-red 

 prawn with orange-coloured eggs [Acantkephyra purpurea). 

 The layer from 800 or 1000 metres downwards may require 

 to be still further subdivided, for certain forms like the larger 

 Acanthephyra with red eggs i^A. midtispina). Notostomus and 

 several fishes and squids have been taken only in the deepest 

 hauls at 1500 or 2000 metres, but we must point out that the 

 deeper parts of the Atlantic were not investigated by us, our 

 efforts being devoted mainly to the upper layers between 1500 

 metres and the surface. We shall, therefore, consider the layers 

 below 500 metres as a whole, referring to some characteristic 

 forms from this bathypelagic region, examining their horizontal 

 and vertical distribution, and discussing the laws which seem to 

 govern their occurrence. 



We have seen that Haecker, in dealing with the vertical 

 distribution of the Radiolaria, recognised a Pandora region 

 from 400 to 1000 metres, and an abyssal region from 1500 to 

 5000 metres ; and this division coincides very well with the 

 two regions characterised, respectively, by the occurrence of 

 Cyclothone signata and C. microdoii and by the two species 

 of prawns. 



Among the medusae a similar correlation is found, Periphylla 

 hyacinthina being most abundant at 500 metres, and Atolla 

 bairdi at 1000 metres. 



No nemertines were taken in depths less than 700 or 800 

 metres, and the fifteen specimens belonging to the genus 

 Planctonemertes, taken at five separate stations, were taken 

 beyond 1500 metres. 



The ostracod Gigantocypris was taken at eleven stations, 

 but only one individual occurred at 500 metres, the remainder 

 occurring in deeper water. Pyrosoma spinoswn was always 

 taken beyond 750 metres, most of the specimens coming from 

 1500 metres. 



