LOWER COASTAL ANIMALS 



123 



Mangrove swamp at Monrovia, Liberia. The upper part of the rootstock of the trees is fully 

 exposed at low tide. 



algal coating of grains of sand, which they will sometimes cement together 

 and place in neat patterns round their holes ( Fig. p. 121). 



No crabs are found living on beaches in the temperate zones, but in 

 the southern regions we find sand shrimps just as in Northern Europe, 

 but different species. 



On sandy beaches in the Tropics, where we were able to get right out 

 into the tidal zone and not merely to its edge, we would have our legs 

 bitten by small creatures, always under the surface. The creatures which 

 thus failed to distinguish between dead animals and live humans would 

 be various isopods (Serolis). They live in symbiosis with some crab-like 

 crustaceans which burrow into the bottom. On sheltered tropical coasts 

 we found various snails crawling on the bottom, which in that case would 

 rarely dry out. Predominant among these were some which in form re- 

 sembled top-shells, except for a small siphon below the shell mouth 

 (Cerithium, etc.). We would sometimes find them in considerable num- 

 bers. 



Snails also predominated in this belt in temperate zones, though here 

 they were quite small forms (Bittium). 



Growing all over the Tropics in the most sheltered places is the vegeta- 



