Handbook of Paleontology 103 



with the degree of salinity of the water in which it is 

 living. 



Experiments have been made with marine animals to 

 determine whether they can be induced to live in gradu- 

 ally freshened water. It was found that almost all 

 species of marine mollusks die if they are brought sud- 

 denly into fresh water but that many species can endure 

 the gradual addition of fresh to salt water until eventu- 

 ally the water has become quite fresh. Nearly related 

 species behave very differently in this respect. The experi- 

 ments have also shown that the powers of adjustment of 

 shore animals to variations in salinity is relatively enor- 

 mous and that animals that live near or below low-tide 

 mark are far less able to accommodate themselves to such 

 changes even though they may be brought about very 

 slowly. The rock shore forms, such as the Acorn Barna- 

 cles (Balanus), Limpets (Patella) and the Purples or 

 Rock Snails (Purpura) and estuarine forms such as the 

 Oysters (Ostraea), Cockles, (Cardium) and Common 

 Mussels (Mytilus edulis) are able even to endure water 

 that is completely fresh; but forms such as the Scallop 

 (Pecten), Abalone (Haliotis), Waved or English Whelks 

 (Buccinum undatum) , Tellens or Tellinas (Tellina) etc. 

 die before the completion of the experiments. A bryo- 

 zoan (M embranipora) has been found living in brackish 

 water that had one-tenth of the normal salinity required 

 by the species. There were minor modifications of the 

 colony as to number and arrangement of cell spines etc. 

 but apparently no detrimental effects. Nevertheless, even 

 though animals may survive under these abnormal con- 

 ditions, each species without doubt has its mean optimum 

 salinity, that is, a salt content of the water which permits 

 the species to reach its full development. For species 



