PELAGIC ANIMAL LIFE 



565 



taken down to a depth of 1200 metres. In northern waters several 

 species have been taken just at that time of the year when the 

 temperature is highest. 



The Aulacanthidae, the Challengeridae, the Tuscaroridae, and the 

 Medusettidae have siHcious skeletons and prefer mainly cold water. 



Fig. 389. 



Acanthometron pelluciduvi, J. Miiller. 



(After Hertwig, from Steuer. ) 



Fig. 390. 



Acanthonidium echinoides, Claparede and 



Lachmann. (From Popofsky. ) 



The Aulacanthidae are spherical, the skeleton consisting of numerous 

 isolated hollow needles, some of which radiate from the centre while 

 other smaller ones are arranged along 

 the surface of the sphere. The great 

 majority of the Aulacanthidae have 

 been found in the north-western 

 corner of the Atlantic (the Irminger. 

 Sea and Davis Straits), and also 

 south of the Cape Verdes, but 

 several species are very widely dis- 

 tributed, for instance Atilographis 

 pcmdora (Fig. 392) taken in the 

 Mediterranean, Indian Ocean, Paci- 

 fic, and also in the Atlantic north 

 and south of the Equator. This 

 species occurs between 400 and 1000 

 metres, and is considered specially 

 characteristic of these depths. One 

 of the best-known species, A ulacantha 

 scolyinantJia (see Fig. 393), is found, 

 like several other radiolarians, in 

 two races distinguished by their 



difference in size. One is a pygmy 0.6 to 1.8 mm. in diameter, the 

 other a giant about 3 millimetres in diameter. At Naples, and during 

 the cruise of the " Valdivia," Haecker^ studied the bathymetrical 



^ V. Haecker, "Tiefsee-Radiolarien," Wiss. Ergeb, " Valdivia''' Expedition, Bd. xiv. (Jena, 1908). 



Vv 



Fig. 391. 



Acantliostaurus 7iordgaardi, Jorgensen (^J-)- 



(From Jorgensen.) 



