56 THE WONDER OF LIFE 



sloping sheli that often occurs, we should find the light 

 becoming gradually fainter and the seaweeds becoming 

 gradually scarcer, and if we could continue to a depth of 

 about 100 fathoms, we should come to the ' mud-line ' 

 where wave-action ceases and the mud sinks quietly to 

 rest. This is near the edge of the continental shelf, and 

 beyond this is the steep slope leading down to the deep 

 sea. 



The shore-area has been divided by Forbes and others 

 into zones : (a) the strictly Littoral or tidal zone, between 

 the tide-marks, with limpets and acorn-shells, periwinkles 

 and dog-whelks, cockles and mussels, sea-anemones and 

 crumb-of-bread sponge ; (&) the Laminarian zone, where 

 the long pennon-like brown seaweeds grow in profusion 

 with sea-urchins and starfishes and nudibranchs ; and (c) 

 the Coralhne zone, with abundance of calcareous Algae, and 

 such animals as ' buckles ' and ' sea-mice'. But shores 

 differ so enormously that these zones are not of general 

 occurrence ; a great deal depends on the gradient, for the 

 shelf may extend out for many miles, or there may be deep 

 water up to the sides of the chfis and no shore at all, as in 

 the Scandinavian fjords. A noteworthy point that can be 

 readily verified concerns the seaweeds. The dominant 

 colour changes as we proceed outwards. Most of the 

 green Alg£e, such as the sea-lettuce {Ulva lactuca), are in 

 the shallowest water ; the brown ones, such as the huge 

 Laminarians, are most predominant further out ; the 

 red Algse, such as Delesseria sanguinea, are especially char- 

 acteristic of the lowest zone of seaweeds. In the brown 

 forms the chlorophyll is masked with a brown pigment 

 (phycophsein), in the red forms with a red pigment (phyco- 

 erythrin), and the point of greatest interest is simply that 



