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Chapter XVII.) Where seaweed is present, the fauna is enriched by the 

 sponge Suberites domuncula, nudibranchs, the spider and hermit crabs, 

 with sea anemones attached to their shells. 



Such muddy bays, exposed to the tidal currents, are employed for 

 the artificial culture of oysters in England, France, and on the Atlantic 

 coast of North America. The oyster, Ostrea edulis, otherwise an in- 

 habitant of solid bottom, adapts itself well to the mud, the brackish 

 water, and to the shallow water. With careful protection from its 

 enemies, it develops a form characterized by quality and size of soft 

 parts, with a thin shell. These oyster beds require constant replenish- 

 ment by brood oysters (oyster spat) from salt water; the fertility of 

 the oysters in brackish-water beds is insufficient to maintain them. 

 These beds, where a wealth of detritus is stirred up by the tides, with 

 its attendant microfauna, are consequently essentially fattening estab- 

 lishments for the oysters. 



Mangrove associations. — A special facies of the estuarine habitat 

 is represented in the tropics by the mangrove beach. 21 The mangrove, 

 with its stilt-like roots and a variety of plants with similar growth 

 habit, especially Sonneratia (Myrtaceae), Avicennia (Verbenaceae) , 

 and Aegiceras (Myrsinaceae) , is world-wide on quiet tropical coasts. 

 The tangle of roots holds debris brought down by rains, and that floated 

 in by the tide, until it decays. In and upon the mud is found a remark- 

 able assemblage of marine, fresh-water, and terrestrial animals. The 

 small actinian Thelaceros rhizophorae is found at the water's edge, in 

 little pools near the mangrove roots. Uncounted millions of crabs creep 

 between and upon the roots, usually adapted in coloration to that of 

 the mangrove bark. The holes of the fiddler crab (Uca) are every- 

 where, descending to a depth of 75 cm. Hermit crabs live on bottom 

 and roots. At ebb tide, ants abound on the mud, scavenging on the 

 remains of marine life, their nests in the mangrove above, marking the 

 extreme height of the tide. Euryhaline lamellibranchs bury themselves 

 in the mud, Psammobiidae and Solenidae from the sea; cyrenids from 

 the fresh-water swamps. Various types of tree oysters attach them- 

 selves to the mangrove roots, in Java, for example, Ostrea mytiloides. 

 The snails Cerithium, Potamides, and Littorina form a series succes- 

 sively more independent of the sea water, entering brackish water or 

 becoming terrestrial. The fish called the mudskipper, Periophthalmus, 

 is widespread on mangrove coasts, living more out of the water than 

 in it. Its eyes are protected against drying (in the absence of eyelids) 

 by a great development of the conjunctival sack. 22 They are placed on 

 the dorsal side of the head, as in many amphibious animals (Fig. 33) , 

 so that vision above the water is possible when all the rest of the body 



