366 University of California Publications in Zoology [Vol.22 



points, and these are noted in a series of reports (Vrolik, et al., 1860- 

 1864; Van Oordt, et al., 1865, 1869). From these records it appears 

 that the lowest salinities occurred during the year 1861 at Niewendam 

 on the Ij. During: that year the salinity averaged 7.67 parts per 1000 

 of sodium chloride (8.39 parts per 1000 total salts). The lowest record 

 for the year was 6.17 parts per 1000 sodium chloride (6.74 parts per 

 1000 total salts). In this year the salinity remained below 9 parts per 

 1000 from May 1 to December 31, and during the year the borers were 

 plentiful in the region. Teredo navalis has survived much lower 

 salinities at Crockett than those recorded at Niewendam. It appears 

 that the salinitj' was never low enough at the latter place to extermi- 

 nate the borers, although it may have reduced their activity during 

 certain periods. 



Resistance to Sudden Changes in Salinity 



Reference to figure 4 shows that the teredos at Crockett experi- 

 enced many sudden changes of salinity prior to the long period of low 

 salinities in May and June. Differences of four to six parts per 1000 

 between the maximum and minimum salinities for the same day occur 

 frequently. There was a drop of seven parts per 1000 on two days in 

 December and in February a drop of thirteen parts per 1000 in three 

 days. The borers survived all the changes which occurred prior to the 

 prolonged period of low salinity in May and June. 



It has been observed, in the aquaria, that teredos which have been 

 forced to plug up their burrows because of reduced salinity revive 

 very rapidly when placed in salt water. One hour is generally sufficient 

 for resumption of activity by all the living individuals, the siphons 

 being extended and boring activity begun by the end of this time. 





