265 



4 parts per 1000. Thus it may be advisable to assume 4 parts ])er 10(10 as the critical 

 salinity in interpreting the results given in table 41. The real stretch seems to have 

 occurred during the period when the salinity was below this point, since, in the piles 

 examined, 50 per cent of the borers were dead at the end of 19 days, 60 per cent at the 

 end of 26 days, and 90 per cent at the end of 35 days. This corresponds more closely 

 with some of the results obtained in the aquaria (see table 38). 



From this interpretation of the results an estimate of one and one-half months of a 

 salinity below 4 parts per 1000 as the period required for the destruction of all the 

 teredos seems reasonable, and has been borne out by subsequent observations. A 

 careful study of the salinity graphs for this and other localities in the upper bay 

 indicate that no teredos have survived a period of stress as great as this. 



During the years 1859-1869, Dutch investigators made observations upon Teredo 

 navalis along the coast of the Netherlands and in the Zuider Zee. Frequent records were 

 taken of the salinity at various points, and these are noted in a series of reports 

 (Vrolik, et al., 1860-1863; Van Oordt, et al., 1864, 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 salinity was never 

 low enough at the latter place to exterminate the borers, although it may have reduced 

 their activity during certain periods. 



Resistance to Sudden Changes in Salinity 



Reference to fig. 99 shows that the teredos at Crockett experienced many sudden 

 changes of salinity prior to the long period of low salinities in May and June. Differ- 

 ences of four to six parts per 1000 between the maximum and minimum salinities 

 for the same day occur freciuentK'. 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 haw been forced to 

 plug up their burrows because of reduced salinity revi\'e 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 normal activity having begun by 

 the end of this time. 



Effect of Hvdrogen-ion Concentration and Dissolved gases 



While temperature and salinity are recognized as factors of prime importance 

 in the study of the relation of a marine organism to its environment, certain other 

 physico-chemical conditions of the water need also to be taken into account. Among 

 these are the amount of dissolved oxygen present, this gas being necessary for the 

 respiration of aquatic organisms; the amount of dissolved hydrogen sulfide, which is 

 poisonous when present in more than minute cjuantities; and the hydrogen-ion con- 

 centration of the water, which is a measure of its acidity or alkalinity. 



The concentration of hydrogen ions is commonly expressed in terms of the 



