206 



MARINE ANIMALS 



live on such supports on the banks of the shallow Holstein sea only 

 2 m. below low tide. The oyster banks of the deeper waters in the 

 North Sea, between Helgoland and the Netherland-Germany boundary, 

 are also situated on coherent sand; they cover a breadth of 15-20 km. 

 and lie at a depth of 34 to 42 m., affording a further example of the 

 arrangement into communities described by Verrill, 43 Petersen, 37 and 

 others. 



The oysters on the Holstein banks are not close set, but spaced 

 about a meter apart, as support is available, and are accompanied by 

 a varied fauna of other mollusks, 44 hydroids, sea anemones, annelids, 

 and crustaceans. Ostrea glomerata, in Queensland waters, contrasts 



(v^a ;•.■ 



Fig. 38. — Distribution of pearl oysters on rock bottom (left) and on sand bottom 



(right). After Herdman. 



with this type in occurring primarily on rocks, and forming reefs, 

 composed of solid masses of oysters which may be more than half a 

 meter in thickness, the uppermost 20-30 cm. being living oysters, 

 placed on the shells of their predecessors. 45 Where the sand or mud is 

 subject to motion, oysters do not flourish, since it deposits on them 

 and buries them; but active movement of the water is necessary to 

 keep the detritus on which they feed in suspension. They are not 

 sensitive to reduction of salinity. 



The pearl oyster banks of tropical seas are also associated with 

 rock bottom. On the coast of Ceylon 46 such banks are found at depths 

 of 10 to 20 m., at places where sand and mollusk shells and organic 

 materials have been consolidated by carbonate of lime, aided by 

 Bryozoa and calcareous algae. There the pearl oysters {Margaritijera 

 vulgaris), attached by byssal threads, are distributed uniformly over 



