INVERTEBRATE BOTTOM FAUNA 



47; 



Several echinoderms occur numerously wherever there are 

 currents. There are quantities of the brittle-stars : Op/iiop/iolis 

 aciileata (see Fig. 335), Ophiocoma nigra, and Ophiura albida. 

 Two species of sea-urchins that live on the hard bottom in the 

 littoral zone are very common among the skerries on the west 

 coast of Norway, namely Echiims esculentiis and Strongylocen- 

 trotiis drdbackiensis. On the other hand, Echinus acittiis and 

 Parcchinus miliaris^ have a different local distribution, to which 

 I shall allude later. All four species 



may be found up to low tide mark. _ t 1%^ 



This is true also of the big dark- 

 brown holothurian Cuciimaj^ia fron- 

 dosa (see Fig. 336), large numbers of 

 which live on the hard bottom among 

 the skerries, and in the outer parts 

 of the fjords, especially where there is 

 a strong current. They fasten them- 

 selves to the rock by means of their 

 suckers, and often have their tentacles 

 stretched out in order to capture pe- 

 lagic organisms, which are afterwards 

 licked off, the animal sticking one 

 tentacle at a time into its mouth. 



Together with the above forms 

 we find a mussel, Lima hians, which 

 is very characteristic of these localities. 

 It is of interest biologically, because 

 it lives within a nest constructed with 

 the assistance of its byssus out of 

 bits of empty mollusc shells, frag- 

 ments of echinids or serpulids, and 

 similar materials ; in fact, no loose 

 substances appear to come amiss. 



Two starfishes are always present, namely Asterias rubens 

 and A. miilleri. There are other species as well, of course, 

 such as worms and serpulids, but they cannot be called particu- 

 larly characteristic. 



Here, too, the lobster {Homartts vidgaris) is equally at home, 

 and may be met with under rocks and stones, occasionally 

 venturing on to sandy bottom. It is distributed throughout the 

 whole littoral zone from a depth of about one metre downwards, 

 a certain proportion of individuals migrating vertically, descend- 



^ In a few localities all these species may be found together. 



Fig. 336. 

 Cucumaria frondosa. Gun. 



