THE ECONOMIC MOLLUSCA OF ACADIA. 



109' 



that buried deepest in the wood, is the largest, and at its extremity bears 

 the two valves, greatly reduced and forming the boring apparatus. 

 Between them occurs the opening through which the slender " foot " 

 can be protuded, an organ corresponding with the "foot" in the Clam. 

 Except for this opening the mantle or '• skin " of the animal is unbroken 

 until the posterior end is reached, where the body terminates in two 

 tubes or siphons, corresponding with those in the Clam. Probably one- 

 of these is for the admission of food and fresh water, and the other for ' 

 the excretion of waste water and the wood which passes through the 

 animal's body. These siphons can be retracted and the burrow quite 

 closed by two little calcareous valves or pallets, borne near the siphons. 

 The latter, when the animal is distended, are at or near the surface of 

 the wood. 



In burrowing, the Teredo generally follows the grain of the wood, 

 but will cross it when necessary to avoid a knot, nail, neighbour's tube. 



Fig. 22.— Teredo navalis. Twice Natural Size. 



or other obstruction. They never pierce each other's tubes. As the 

 latter are formed, they are lined by a calcareous layer, more or less- 

 thick, and firm enough to allow large pieces of it to be removed entire 

 from the wood after the animal is dead. It is still doubtful what 

 mechanism the animal employs in burrowing. Many naturalists think 

 the valves, armed bj' numerous fine constantly growing teeth, form the 

 apparatus, and that by them the animal eats, or rather files, its way 

 onward. One observer states, that by breaking away the side of a 

 burrow, he has actually seen this operation going on; others claim that 

 the foot accomplishes the work, by the same unknown means by which 

 certain limpets can excavate rocks. It is pretty generally agreed, how- 

 ever, that the Teredos do not get nourishment from the wood, but bore- 

 it simply to form a home. Their food consists of minute animals and. 

 plants, taken into the body through one of the siphons. 



