476 



DEPTHS OF THE OCEAN 



are always raised above the level of the sea-floor, the one being 

 for supplying food and water, and the other for voiding excre- 

 ments. The Spatangids get their nourishment down in the 

 sand by means of their remarkably shaped mouth-feet, and 

 through the rapid vibrations of the spines, some of which are 

 specially adapted for the purpose, they keep the water circulat- 

 ing in the holes where they lie, and so obtain oxygen for breath- 

 ing. Astropecten has a row of small spines along its arms, which 

 vibrate in similar fashion, and cause a circulation of water round 

 its body. The tubes of the worms are almost invariably directly 

 connected by an opening with the level of the sea-floor. 



Among the higher crustaceans inhabiting the sandy bottom 



we get one or two 

 species of swimming 

 crabs {^Portumts, see 

 Fig- .339)- They har- 

 monise in colour with 

 the variations in the 

 colour of the bottom, 

 and are thus enabled 

 to escape notice when 

 motionless. Their 



name is derived from 

 the terminal joint of 

 the fifth pair of swim- 

 merets, which is ex- 

 panded and paddle- 

 shaped, so that they are able to swim upwards. During the 

 cruises of the " Michael Sars " in the North Sea one of these 

 swimming crabs {Porttnuis depiLrator^ was found hanging in 

 the drift-net, and numbers of young crabs of the same species 

 were captured in the plankton net. These forms must, 

 nevertheless, be regarded as genuine bottom animals ; I have 

 observed that they can even burrow down into the sand for 

 a short time, but never remain there long. 



One of the most characteristic forms of the littoral zone is 

 the common edible crab. Cancer pagiirus, which is not so 

 particular as the lobster regarding the nature of the bottom, 

 being as much at home on sand as on rocks. Cancer pagiiriLS 

 goes farther up the fjords than the lobster does, but they both 

 are undoubtedly littoral animals, occasionally found close up to 

 low-tide mark, and occurring exceptionally below the lower 

 limit of the littoral zone. 



Fig. 339. 

 Portiaius depurator, L. 



After Bell. ) 



