CHARACTERISTICS OF THE BORERS 969 



Chakacteeistics of the Borings 



Features of the hole itself are often characteristic of the type of borer 

 producing it and even positive proof that the borer entered the material 

 when it was in a well advanced stage of induration. Thus the club- 

 shaped bores^ with very small openings made in the rock by the more 

 highly specialized pholads, indicate a slow progress and enlargement with 

 groT^d^h in hard material, in contradistinction to bores of more nearly 

 uniform diameter, with rather large openings, made by species of Zir- 

 plicea in mud or clay. The shape of the bottom of the hole of sand- and 

 mud-boring pholads is likely to be bluntly pointed rather than subspher- 

 ical, as in the case of the rock-boring species of pholads. Sharp mark- 

 ings on the wall of the bore caused by the rasping of the points of the 

 shell, as in the case of the holes of Pholadidea, also indicate that the hole 

 was originally drilled into solid rock. The close conformation of the 

 shape of a mytilid bore to the shape of the shell itself, without the defor- 

 mation of the shell, fixes the responsibility for making the hole upon the 

 mytilid. 



Influence of Environment 



There is a definite correlation between the rock-boring habit and a loca- 

 tion upon a more or less open coast where there may be ready access to a 

 fresh supply of plankton food material fresh from the open sea and to 

 water tolerably free from silt and of a proper salinity and oxygen content. 

 These optimum conditions seem to be much more imperatively demanded 

 by the rock-borers than by many of the mud- and sand-burrowers. In 

 response to this correlation, we find the rock-borers usually absent from 

 estuaries and embayments, where proper food can not be secured and 

 where the deposition of silt may be heavy. Moreover, rocks and ledges 

 which may be utilized by boring animals are found more frequently on 

 the open coast than in embayments. 



Unfortunately but little data is at hand concerniiig the depth in the 

 sea at which boring animals may live. The plankton food supply ex- 

 tends to such a great depth as probably not to be a determining factor. 

 Temperature is a very potent factor in limiting the distribution of marine 

 animals, but would probably not be effective alone in limiting the range 

 of rock-borers within 50 fathoms. Salinity remains nearly constant 

 except near the outlets of rivers. Conditions of thorough aeration and 

 the rapidly i-eplenished food supply of the tidal zone seem to have at- 

 tracted many animals of the littoral fauna, while the activity of the water 



