666 



DEPTHS OF THE OCEAN 



whose upper limit in the north, at any rate, may be put at 

 above lOO metres.^ Now, if we calculate the depth to which 

 the rays of the sun penetrate, after passing through the same 

 distance in the water, assuming always that the rays are direct 

 and that the rate of absorption is the same, we find that the rays 

 will have passed through the same distance to reach a depth of 

 500 metres in lat. 50 N,, that they will pass through to reach 

 650 metres in lat. 33° N., or 300 metres in lat. 67^ N. 



The transparency of the water, however, varies greatly in 

 different regions. If we take the results of previous observa- 

 tions during different expeditions, we may set down the visible 

 depth in the open sea as being roughly 50 metres in lat. ;^2>" ^m 

 40 metres in lat. 50° N., and 25 metres in the Norwegian Sea 

 in lat. 67° N. Taking this into consideration, we find that 

 there will be the same intensity from the rectilinear rays — 



In lat. 33° N. at about 800 metres. 



,, 67° ,, 200 ,, 



The red and black animal forms, therefore, as has been 

 found in the investigations I have just described, have an upper 

 limit in the different waters which corresponds everywhere with 

 the same intensity of light. 



Very interesting also is the fact that certain dark bathy- 

 pelagic forms appear as varieties differing in the intensity of 

 their colours. Broch from his study of the "Michael Sars " 

 collections thus recognises four varieties of the deep-sea medusa 

 Atolla bairdi: (i) stomach alone containing pigment; (2) 

 peripheral muscular belt also pigmented ; (3) the brown pigment 

 distributed also on the lower side of the bell, while gonads are 



^ Sir John Murray reports that in Upper Loch Fyne, in Loch Etive, and in some other sea- 

 lochs of the west coast of Scotland, which are cut off from the ocean by submerged barriers, red 

 prawns and other red crustaceans are very numerous in depths of 50 to 70 fathoms (about 270 

 to 310 metres) ; for example Nyctiphaiies {Megaiiyctipliattes noi-vegica), both adult and young, 

 can always be captured in these lochs by dragging nets one or two fathoms above the bottom. 

 This species possesses ten phosphorescent organs : one pair in the eye peduncle, two pairs on 

 the under side of the thorax, and the remaining four in the median line of the abdominal seg- 

 ments. Sir John believes that these organs are used as a kind of " bull's eye lantern," and 

 enable the Nycliphanes to see and pick up the minute particles of organic matter which are 

 settling on the bottom-deposits. Many specimens of this species were kept in aquaria for a 

 considerable period, and were observed to light up and shut off their phosphorescent organs at 

 will. The surface layers of water in these Scottish lochs are much less saline than the deeper 

 layers, and contain much suspended matter, so that the penetration of light is much obstructed. 

 Besides Nydiphanes other red or red and transparent crustaceans are always to be captured in 

 the deeper water-layers of the Scottish sea-lochs, such as Calainis Jinniarchicus, Eticha-ta 

 norvegica, Conckaria elegans, Boreophatisia raschii, Pandalus anniilicornis, Pasiphcea sivado, 

 Cratigon allniani, Hyppolyte securifrons, etc. (see Murray, Scot. Geogr. Mag., vol. iv. pp. 353-6, 

 1888 ; Coiitptes rendits des Seances dii jine Congres international de Zoologie, Leyde, 1895, 

 p. 107). 



