402 University of California Publications in Zoology [Vol. 22 



DESCRIPTION OF THE PALLETS 



The pallets of Teredinidae are peculiar structures, characteristic 

 of this family of boring mollusks. In Teredo navalis they are small, 

 spatulate objects, consisting of a relatively broad blade, flattened on 

 the inner surface, somewhat convex on the outer, into the base of 

 which is inserted a typically narrow, cylindrical stalk. The stalk 

 is calcareous and rigid. The blade also has its proximal portion 

 calcified, but ends in a darker colored chitinous tip, which is more or 

 less deeply excavated. 



The pallets are attached at the posterior end of the animal, on 

 each side of the siphons, being inserted beneath a muscular collar, 

 which covers the stalk and partly conceals the blade when the struc- 

 tures are retracted (pi. 19, fig. 1). Ordinarily the siphons are 

 protruded from the opening of the burrow for purposes of feeding 

 and respiration; but if the animals are disturbed, or environmental 

 conditions become unfavorable, the siphons are immediately with- 

 drawn and the pallets are thrust into the opening to act as a plug 

 (pi. 19, fig. 2), thus effectually keeping out intruders and affording 

 protection for a considerable period of time from reduced salinities 

 or other unfavorable conditions. The chitinous tips of the pallets, 

 which are slightly flexible, doubtless permit of a more efficient closing 

 of the burrow than would be the case if the entire structure were 

 rigidly calcified. 



In the early stages of the teredo's life in the wood, the nacreous 

 lining of the burrow is partly visible at the surface, giving the appear- 

 ance of a carefully sculptured doorway, with definite grooves dorsad 

 and ventrad for the passage of the two siphons (pi. 19, fig. 2). With 

 increasing age, however, this exposed part of the lining of the burrow 

 becomes eroded or dissolved away; the posterior end of the animal 

 is gradually withdrawn, the larger size of the pallets enabling them 

 to close the tapering burrow without extending to the surface of the 

 wood. Thus, in an older specimen, when the siphons are retracted, 

 the only indication of its presence is an inconspicuous opening, as 

 seen immediately to the right of the principal figure, plate 19, figure 2. 

 The siphonal end of the animal, however, is never withdrawn in this 

 way more than a few millimeters from the surface of the wood ; and 

 the nacreous lining is normally kept in good repair, forming a sheath 

 into which the pallets may tightly fit when extended. 



