1923] Miller: Variations in the Pallets of Teredo navalis 403 



GROWTH CHANGES 



We find, as we should expect of structures so useful in the economy 

 of the animal, that the pallets are formed very early in the boring 

 life. The larval teredo has hardly initiated boring activities when 

 rudimentary pallets come into evidence, and these become functional 

 when the animal is only one or two millimeters in length. In these 

 early pallets the blade is short and. broad, and the stalk relatively 

 long. But by the time the animal has reached an age of one month 

 the pallets have assumed normal proportions, and, so far as our 

 observations go, no changes ensue thereafter as a result of growth 

 alone. 



A small series of pallets selected for age is shown in plate 19, 

 figure 4. The factor of individual variation could not be entirely 

 eliminated even in this limited group ; but it is rather evident that 

 such changes as occur after the first month are due primarily to 

 effects of wear and a gradual darkening of the chitinous tip, which 

 generally becomes more pronounced in the older pallets. Indeed, 

 the pallet estimated to be one month old more nearly approaches what 

 we consider the type of navalis than do the others in this series. 



In the selection of our material to illustrate variation, pallets 

 adjudged to be less than one month old were eliminated, in order to 

 avoid the error of including as variates specimens which might not 

 yet have reached their normal proportions. 



The age of pallets was estimated by counting the ridges on the 

 corresponding shells, as explained in our earlier paper (1922, pp. 

 296 ff.). 



