PELAGIC ANIMAL LIFE 



571 



(boreal), viz. Aglantha digitalis (see Fig. 399), which sometimes plays an 

 important part in the pelagic life of the Norwegian Sea ; in the North 

 Sea Hensen fell in with a shoal of these medusae which he estimated at 

 23^7 billions of individuals. As mentioned by Haeckel, it is character- 

 istic of this form that it suddenly appears in enormous quantities for 

 some days and then suddenly disappears for some months. 



As rare visitors in the north may be mentioned, Pantachogon 

 haeckelii, Pectyllis arctica, and Crossota norvegica} Other species are 

 strictly limited to the warm zone of the ocean, which may be said to 



f^. _.,..._ 



Fig. 403. 



Halicreas rotiindatum, Vanhoffen (^'). 



(From Vanhoffen.) 



Fig. 402. 



Agliscra ignea, Vanhoffen (f ). 



(From Vanhoffen. ) 



reach the 40th or 50th degree of latitude, where we Hnd some small 

 forms living entirely in the upper layers of the Atlantic and Indian 

 Oceans, as for instance RJiopalonenia velatuni, Aglaura hemistoma, 

 and Liriope tetraphylla (Fig. 400) ; they are devoid of colour or 

 only faintly tinted, some of them being only a few millimetres in 

 diameter. Others are genuine deep-sea forms, found only below 600 or 

 1000 metres. Crossota brunnea (Fig. 401) is dark brownish, Agliscra 

 ignea (Fig. 402) is a flaring red, and Halicreas rotundatum (Fig. 403) is 

 distinguished by bright red markings. 



The Narcomedusa; are oceanic forms, including some small colourless 

 surface forms and strongly tinted (brown) deep-sea forms. 



1 This species was taken by me in a deep haul in the Norwegian Sea, and Vanhoffen placed 

 it very near to the tropical species Crossota brunnea, see Wiss. Ergeb. " Valdivia" Expedition, 

 Bd. 3, 1902 ; and "Die Fauna und Flora Gronlands," Grdnland Expedition (Berlin, 1897). 



