82 THE BIOLOGY OF THE SEA-SHORE 



salinity may quite well be more important than temperature. 

 On the other hand, the close dependence of off-shore forms 

 on a definite degree of salinity is well shown by an unusual 

 extension, in 191 7, in the distribution of the Shipworm 

 {Teredo diegensis) in San Francisco Bay (Barrows, 1917). 

 A salinity of at least ten parts per 1000 seems to be 

 required for the existence of this pest, at temperatures 

 ranging from 6° C. to 19° C. An increase in salinity of as 

 little as three or four parts per 1000 above the minimum 

 causes the worms to increase considerably in numbers and 

 stimulates their activity. On the other hand, during a 

 season of heavy rainfall, when much fresh water enters the 

 bay, they may be killed off entirely, though reinfection may 

 occur during the next summer and autumn, and in dry 

 seasons the shipworms increase so as to become a very 

 serious pest. 



Phenomena of an essentially similar nature to those 

 which cause the variation in salinity and temperature of 

 coastal waters in general, are regularly affecting, in a similar 

 manner, the conditions of life in shore pools deserted by the 

 tide. The smaller the volume of water in the pool, the more 

 intense are the effects. Under the influence of a strong sun 

 the density of the water increases rapidly, as does also the 

 temperature. Rainfall, obviously, has the reverse effect. 

 According to Issel (1918) the salinity of rock pools may vary 

 from a little above 2 or 3 grams per litre of dissolved salts 

 to over 300 grams of salts per litre. The same writer 

 has made some interesting observations on the behaviour 

 of the flagellate Protozoan Carteria suhcordiformis , and that 

 of the Copepod Harpacticus fulvus in shore pools under 

 evaporation. The former is sometimes present in such 

 numbers as to colour the surface of the pools green. By the 

 time the water in these pools has become strongly saline, 

 about 1*125, these animals have come to rest and show 

 no sign of Hfe. They have entered upon a period of latent 

 life and may survive in this condition for 2 or 3 weeks, 

 recovering on the salinity of the water being reduced again 

 to normal. Herdman (1892) has made similar observations 



