8 N. J. Agricultural Experiment Stations, Circular 139 



edges of the shells are pushed far apart and never touch as they do in 

 the clam ( fig. 5 ) . 



Everyone who has seen a clam on the half shell has noticed the two 

 rounded pinkish white adductor muscles, one in front and one behind, 

 by which the shells are closed. In the pileworm these muscles do not 

 work together as they do in the clam, but alternately, so that the shells 

 instead of opening and closing as do those of -the clam, rock backward 

 and forward on the knobs where they are in contact with each other. 

 The muscle at the hinder end is much stronger than that in front, 

 and has a greater leverage, so that it is able to exert a considerable 

 force. 



Fig. 5. Anterior, or Front End of Teredo navalis Showing Shells 



Enclosing the Circular Foot 



f , foot ; t, the rows of rasping teeth which are carried on the front edge 



of the two shells 



How Burrows are Made 



The outer front surfaces of the shells bear several rows of rasping 

 teeth which point backward ( t, fig. 5 ) . Consequently, when the hinder 

 adductor muscle contracts, the front edges of the shell are sprung 

 apart with such force that the teeth scrape off little shavings of the 

 wood. The foot, which is circular and hollowed out, acts as a sucker 

 and keeps the front edges of the shells, which bear the teeth, tightly 

 against the end of the burrow while the animal is boring ( fig. 5 ) . As 

 the mollusc grows it rasps away more and more of the wood, length- 

 ening and enlarging the burrow. The shavings pass through the 

 digestive tract, where they are held for some time, and then cast to 

 the outside through the excurrent siphon. 



As its burrow is extended the pileworm lines the walls with a thin 



