472 



DEPTHS OF THE OCEAN 



grows. Litto7'ina littorea and L. obtusata again are found in 

 greatest abundance wherever there is shelter, while Nacella 

 pelliicida generally lives on the blades of Laminaria hyperborea. 

 In the sheltered haunts of Lmnmaria saccharina and L. 

 digitata, particularly on the first named, we find the brittle-star 

 OpJiiothrix fragilis, while the localities with L. hyperborea have 

 evidently no attractions for it ; the blades of L. saccharina, too, 

 are much patronised by the bryozoan Aetea. Asterias glacialis 

 (see Fig. 334) also prefers sheltered localities. Why there 

 should be these apparently capricious affections is as yet un- 

 known, but it may be that in undisturbed waters there are 

 higher temperatures during the summer, and that consequently 

 various influences are brought to bear upon the organisms at 

 one stage or another of their 

 lives. 



The most typical localities 

 of this kind are met with as 

 portion of the a rul.e in sounds amongst the 

 skerries, where there is a more 

 or less strong current, which 

 carries away the finer particles 

 of mud that would otherwise 

 settle, and leaves only large 

 fragments of shells and similar 

 debris. On the hard bottom 

 there are usually numbers of 

 both attached and unattached 

 forms, chiefly consisting of bryo- 

 zoans, hydroids, especially the 



genus Ttibitlaria, and ascidians. The coral Alcyoniiim digitatiim 

 too is often plentiful,^ generally attached to large empty mussel 

 shells or stones. The empty mussel shells are also patronised 

 by big colonies of the serpulid Pomatoceros triqueter, which 

 however is just as much at home on the rocks up to the very 

 shore. There are, besides, Anoinia ephippittm, Chiton cinereus, 

 Tectura virginea, Buccimun nndatiwi, and several others, some 

 sedentary, and others, like the chitons and Tectiira, able to 

 move about from one place to another ; as well as Mytilus 

 77todiolus, though this mussel is far more plentiful inside the 

 fjords, and Gonactinia prolifera. 



Fig. 335. 

 OphiopJiolis aculeaia, L. 



^ This form may even be found up to hiw-tide mark, where there are strong currents, as for 

 instance in narrow shallow sounds. 



