INVERTEBRATE BOTTOM FAUNA 529 



and a few other species of Tellina, Verms fiucttwsa, Cardium ciliatmn, 

 C. grojilandicum, Thracia trwicata (rarely found in the boreal region), 

 Pandora gladalzs. Brachiopods : Rhynchonella psittacea (see Fig. 371), 

 TerebraUdina spitsbergensis. Echinoderms : Asterias lincki, A. panopla, 

 A. gr'bnlandica, A. Jiyperborea, SticJiaster albulus, Ctenodiscus crispatus, 

 Ophiopleura borealis, OpJiiura nodosa, Amphiura sundevalli, Ophiopus 

 arcticus, Gorgonocephalus eucneniis and G. agassizi, Antedon eschrichti, 

 A. prolixa, Cuciimaria niinuta, C. glacialis, 

 Eiipyrgus scaber, Trodwstoma boreale, Ankyro- . — ' -i>.^ 



derma jeffreysi, Chirodota Icevis, Myriotrochus 

 rinki. Decapod crustaceans : Sclerocrangon ^ 



ferox, S. boreas, Sabinea septemcarinata, Hippo- ^^^^^^^ -^-^ V\ 

 lyte turgida and H. spinus, BytJwcaris payeri, 

 Idotea entonion. Two species of pycnogonids, 

 Ny nip] ion robustuni and N. Jm'tipes, are very ^ig. 371. 



abundant in the arctic region ; the former is Rhynchonella psittacea, Chemn. 

 largely a deep-sea form, which descends far (After G. o. Sars. ) 



down into the cold area of the Norwegian Sea 



deep basin, whereas N. liirtipes belongs more to the banks and plateaus. 



Both species were trawled by the "Michael Sars" on the Jan Mayen 



plateau, showing that they may be abundant in shallow waters also. 



The largest pycnogonid of the Norwegian Sea is Colossendeis proboscidea, 



found both on the slopes of the deep basin and on the banks. 



There are also several other species of Nyniphon, such as N. elegans, 



N. macronyx, and N. gracilipes, which are 



common arctic forms. The hydroids have 



comparatively few purely arctic species, 



though the magnificent large Tubularia 



regalis is one that deserves special notice ; 



in congenial localities like the Bear Island 



shoal and the banks of Jan Mayen it forms 



regular thickets on the bottom. Among 



ascidians Dendrodoa {Styela) aggregata (see 



Fig. 372) is a very characteristic arctic form, 



and is often found in little colonies com- 



^^^ , ,5 posed of a number of cohering individuals. 



l&^jl,^^' Another characteristic though rarer species 



*jj*B* jg Chelyosoma madeyanum, easily recognis- 



iG. 372. ^]_jjg owing to its extremely flattened shape 



Dendrodoa a^pre^ata, Rathke. 1,1 • . 1 • 1 •. r • 



N^t. size. ^"" the squares mto which its surface is 



divided. Ciona intestinalis, one of our com- 

 monest boreal forms, occurs in the arctic tracts as a distinct variety 

 {longissinia\ The compound ascidians are represented by several 

 species, amongst which the tuberous Synoicmn incrustatuin, whose 

 surface is encrusted all over with grains of sand, may be easily recog- 

 nised. Other forms are Molgula retortiforinis, Amarouciuni niutabile 

 (tuberous and of a reddish-violet colour), and Sarcobotrylloides aureuni. 



The second category of forms in the arctic region is made Arctk-boreai 

 up of those which are at the same time extensively distributed ^°''""- 



k. 



