4 BULLETIlSr 122, UNITED STATES NATIONAL MUSEUM. 



function of protection is almost completely lost. Many interesting 

 hints of how this change of structure has come about present them- 

 selves when one follows the embryologic history of these animals, for 

 in their infancy they are i3rovided with a tj'pical bivalve shell, which 

 soon becomes modified for its main mission, an organ for excavating 

 the protective burrow necessary for the existence of the adult animal. 

 Other links in the evolutionary chain through which shipworms may 

 have passed are suggested by the nearly related family Pholadidae, 

 whose members are also borers, but mostly mud or rock excavators, 

 though some also affect wood. 



Much speculation and considerable controversy has been printed 

 regarding the methods employed by borers in excavating their bur- 

 rows. In this investigation I have carefully examined under the 

 microscope most pieces of wood that have come to hand, to see if I 

 could find traces of the chisel marks of the shell. Practically all of 

 these examinations have proved disappointing, but a piece of palmetto 

 timber forming the home of Teredo {Teredops)fio)'idana showed 

 beautifully the chipping accomplished by the tiny chisels of the shell, 

 and I am led to believe that most of the exogen wood affected, on ac- 

 count of its hardness requires so many raspings that the completed 

 burrow becomes quite polished by the repeated tooling to which it is 

 subjected, and thus fails to present the teeth marks shown in the 

 easily cut softwood of the endogen palm. There is also a specimen 

 of Parapholas califomica Conrad in the collection of the United 

 States National Museum (Cat. No. 151216), which shows satisfac- 

 torily the teeth marks of the shell in the burrow, which is in volcanic 

 tuff. 



There seems to be an additional popular concept for the existence 

 of the burrow, and that is that the ship worm makes this just inci- 

 dentally while he is feeding upon the wood. That the shipworm ac- 

 tually ingests the sawdust made by the teeth of the shell has been 

 conclusively demonstrated by a number of observers. That he passes 

 all of the wood through the alimentary canal is open to question, for 

 many mollusks are known to be able to regurgitate undesirable ma- 

 terial through the incurrent siphon by spasmodic contraction of the 

 mantle. Even where sawdust has been swallowed, it is questionable 

 whether the secretions of the digestive glands are capable of chang- 

 ing lignose — that is, wood, into a crystalloid carbohydrate, which can 

 be absorbed. It is possible that the shipworm may simply indulge 

 in a partial meal of this kind to have the comfortable feeling of a 

 copious repast. The main food of shipworms, as that of all other 

 bivalves and many other mollusks, consists of the countless minute 

 free swimming plants and animals, collectively called plankton. 

 These creatures are beaten down by the cilia of the gills as the water 



