281 



A small series of pallets selected for age is shown in figure 113, 4. The factor of 

 iiuli\i(lual \ariation could not be entirely eliminated e\en 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 be- 

 comes more pronounced in the older pallets. Indeed, the pallet estimated to be one 

 month old more nearly approaches wliat we consider the type of navalis than do the 

 others in this series. 



In studying variation in the pallets, it is important that physiological factors 

 should be separated so far as possible from purely physical ones. Of the latter there 

 are those already mentioned of accident and erosion. The pallets are, from the nature 

 of their function, much exposed to unfavorable circumstances. In times of stress 

 they form the barrier against undesirable and often lethal conditions. They are much 

 subject to wear from being frequently thrust into the constricted end of the burrow. 

 While in such position they may be eroded by organic acids present in the water 

 or the wood. If a foreign object, a piece of floating driftwood, for example, comes 

 violently into contact with the surface of a timber in which teredos occur, the pallets 

 are likely to be broken. 



In figure 113, 4, it will be seen that even a pallet four months old may show very 

 decidedly the effects of wear. In figure 113, 5, we have compared a normal and rather 

 typical pallet from Goat Island with an older pallet from the same locality which 

 illustrates the maximal effect of age and erosion, the original form of the distal portion 

 of the blade being entirely lost. The secondary calcareous accretions about the stalk 

 may be pathological; similar accretions have been noted in other cases, as in figure 

 114, i, g, but are infrequent. 



The pallets may also be secondarily modified by the nature of the burrow. If 

 the course of the burrow deviates suddenly near the opening, the pallets will tend 

 to be asymmetrical, with the stalk curved and inserted at one side of the median line, 

 as in If, figure 114. If the burrow be straight for some little distance from the surface, 

 expanding gradually and regularly, the pallets will usually be well formed, elongate, 

 and tapering (fig. 114, 2 b). If the burrow expands suddenly, owing to an unusually 

 rapid growth of the animal, or if the opening becomes enlarged, as may easily occur 

 in soft wood, the pallets are likely to be broad and blunt in appearance (fig. 114, 3 e). 

 In other words, if they are to perform their function aright, the pallets must fit the 

 burrozv: hence they will be influenced by the factors determining the nature of the 

 latter, such as rapidity of the growth of the borer, hardness of the wood penetrated, 

 and the degree of crowding, which often determines the course of the burrow. 



Excluding the foregoing factors, which may be spoken of as adventitious, we find 

 yet a number of variations which we may term physiological, in that they are indi- 

 recth' rather than directly produced. These may be separated into two types, which 

 for convenience we shall term individual and environmental, the former considered to 

 be produced by obscure causes bound up in the physiology of the individual organism, 

 the latter affording some evidence of correlation with conditions of the environment. 



As an illustration of individual variation, we would call attention to differences 

 which frequently appear between two pallets of the same pair. Examples of this are 

 shown in figure 113, 3. The differences between the members of each of the three 

 pairs of pallets figured are so marked that they would not have been suspected of 

 belonging to the same pair, had we not actually removed them from the animals. 

 Variations of this sort cannot of course be attributed to factors in the environment 

 (exclusi\e of those of an adventitious nature), which would be expected to operate 

 alike on the two members of each pair; rather do they represent some unknown factor 



