1923] Miller: Variations in the Pallets of Teredo navalis 405 



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 figure 6 c, 

 plate 20. 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 (pi. 20, fig. lb). 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 appear- 

 ance (pi. 20, fig. 8 e). In other words, if they are to perform their 

 function aright, the pallets must fit the burrow; 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 pene- 

 trated, 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 indirectly rather than directly pro- 

 duced. 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 plate 19, figure 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 (exclusive 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 of the 

 biochemical or other aspect of the organization of the animal which 

 tends to asymmetry. 



We have found in our study of pallets that the range of this type 

 of variation is so great as to render it extremely difficult to recognize 

 variations resulting from ecological factors. It has been noted, how- 

 ever, that pallets from the upper bay tend to have a heavier peri- 



