530 DEPTHS OF THE OCEAN 



over the boreal parts of the Norwegian Sea, and are thus just 

 as much boreal as purely arctic ; I append a short list : — 



Molluscs : Lepeta ccBca, Margarita grdnlandica and M. Jielicina, 

 Lnnatia grdnlandica, Littorina rudis, CylicJma alba, Leda pernula, 

 Modiolai'ia Icevigata and M. nigra, Astarte {Nicania) banksi with 

 varieties, Astarte compressa, L. ( = elliptica, Br.), Mya truncata (chiefly 

 arctic, whereas ilf. arenaria is the typical boreal form), Saxicava 

 arctica, Pecten hoskynsi, Portlandia frigida. Echinoderms : Strongylo- 

 centrotus drdbachiefisis, Pont aster tenuispinus, Echinaster {Cribrelld) 

 sanguinolentus, Solaster papposus (occurs as a rule in the arctic region 

 as a distinct variety, ^. affinis), Pteraster inilitaris, Ophiura sarsi and 

 O. robusta, Ophiocten sericeuni, OphiopJiolis aculeata, OpJiiacantha 

 bidentata, Ophioscolex glacialis, Cucuinaria frondosa, P solus phantapus. 

 Decapod crustaceans : Pandalus borealis, Hippolyte gaimardi, and 

 H. polaris, Pagurus pubescens, Hyas araneus and H. coarctatus. 

 Isopods : Munnopsis typica. Pycnogonids : Nymphon grossipes (and a 

 few other species of Nymphon). Ascidians : Pelonaia corrugata, Styela 

 rustica and 5. loveni, Styelopsis grossularia, and Ascidia prunmn. Worms : 

 a number of species of HarmotJioe, Lumbrinereis fragilis, Onuphis 

 conchy lega, Nereis pelagica, Arenicola piscatoruni {marina), Owenia 

 assimilis, Nicolea zostericola, Thelepus circinnatus, and Terebellides str'bmi. 



These forms are very interesting biologically, as they show 

 to how great an extent the same species is able to adapt itself 

 to different natural conditions. Many of them ^ are quite 

 common in the littoral zone of the boreal region, where during 

 a large portion of the year the temperatures are comparatively 

 high, and yet they are also to be found in arctic tracts, where 

 temperatures are all the year round below o C, or at any 

 rate not more than a few degrees above o*" C. during a very 

 short period. Others, again, are more consistent, as they 

 inhabit only the greater depths of the boreal region, where 

 throughout the whole year the temperature is fairly uniform and 

 comparatively low (though never sinking below 6^ or 7 C), 

 whereas in the arctic region they exist in shallow water ; thus 

 on the Norwegian west coast we find the mussel Porllandia 

 frigida, the brittle-star Ophiacantha bidentata, and the prawn 

 Hippolyte polaris (see Fig. 373) only as a rule beyond 100 

 metres, whereas in high arctic latitudes they may be met with 

 at a depth of about 10 metres. The species included in this 

 second category do not all by any means show the same dis- 

 tribution throughout the arctic region ; some forms occur every- 



^ Margarita grdnlandica and M. helicina, which both occur in the boreal laminaria belt, 

 Saxicava arctica, Strongylocentrotus drobachiensis, Echinaster sanguinolentus, Ophiopholis 

 aculeata, Cucumaria frondosa, Hippolyte gaimardi, Pagurus pubescetts, Hyas, Styela rustica, 

 Nereis pelagica, Arenicola, Nicolea, etc. 



