256 



salinity of 9 parts per 1000 as in higher saHnities. In salinities below 9 parts per 1000, 

 the per cent of functioning individuals tiecomes less, and decreases more rapidly as 

 the salinity is decreased, until at 3 parts per 1000 there arc no siphons extended. 

 The curve descends rather gradually from 9 to 6 parts per 1000 and then slopes 

 abruptly down to 4 parts per 1000. This rapid decrease in the number of siphons 

 extended in salinities below 6 parts per 1000 indicates that, below this point, the vital 

 functions are abruptly interfered with in a large number of individuals. 



The reduction of activity in these lower salinities is also indicated by the cessation 

 of boring activity. The degree of this activity was estimated from the amount of 

 chips ejected which collected on the bottom of the aquarium. In the salinities of 

 7 parts per 1000 and above, boring was found to be very active, in 6 parts per 1000 



10 11 12 



13 14 15 



Salinity in parts per 1000. 



Fig. 98. Graph representing length to which siphons of Teredo navalis are extended in water of 

 various salinities. 



slightly less, in 5 parts per 1000 very rare, while in 4 jiarts per 1000 it was never observed 

 in the aquaria. 



There was also an accompanying change in the sensitivity of the siphons, these 

 retracting rapidly in the salinities of 6 and 7 parts per 1000, moderately in 5 parts per 

 1000, and slowly in 4 parts per 1000. 



Again, there was a marked change in the length to which the siphons were ex- 

 tended, which is illustrated in figure 98. This curve shows a gradual decrease in the 

 length of the siphons with decrease in salinity, the decrease per part per 1000 becoming 

 greater as the salinity decreases. 



