269 



CHAPTER XVII 



THE BIOLOGY OF TEREDO NAVALIS (Continued) 



Variability and Environment 



The shell and pallets of Teredo are structures which are subject to a wide range 

 of variation, which, as mentioned in an earlier chapter, has Ijeen a cause of much 

 confusion in the classification of the group. It is of interest, therefore, to consider 

 the range of such variations in the case of Teredo navalis and the extent to which they 

 may be referred to known conditions of the en\'ironment. Such a study in the case 

 of one species sheds considerable light on the general problem of the taxonomy of 

 this group. 



Variations in the Shell 



There is scarcely a detail of the structure of a Teredo shell which cioes not exhibit 

 variability. The outlines of the shell and the relations of the various parts to each 

 other are frequently markedly dissimilar in different specimens. This is particularly 

 true with reference to the auricle, which may be reduced or very prominent, rounded 

 or quadrate, or elongate and reflected; its position varies between posterior and 

 postero-dorsal. Other marked variables are the prominence of the ridges on the 

 anterior and median lobes and the width of the spacing between them, details of the 

 serrations on these ridges, width of the anterior median denticulated area, size of the 

 angle formed at the junction of anterior and median lobes, degree of convexity of the 

 shell, and thickness and color of the periostracum. On the interior of the shell, con- 

 siderable variation occurs in the relative length and breadth of the apophysis, and in 

 the width of the shelf produced by the slight overlapping of the inner edge of the 

 auricle upon the median portion. 



To treat in detail of all the deviations observed in these studies would be a prac- 

 tical impossibility, as well as inconsistent with our limited knowledge of the conditions 

 which occasion them. We have accordingly confined attention to those variations 

 which are at once the most salient and the most easily described or expressed in terms 

 of numerical coefiicients. Such are the variations occurring in numbers of ridges, in 

 length of auricle, in certain details of surface sculpture, and in color. These will be 

 considered in the following pages in relation to what seem to be their principal 

 environmental antecedents. 



In order that any effect of en\uronment in producing variation might be observed 

 in its maximal expression, we selected for this study specimens of Teredo from localities 

 most distinctly characterized by the physical differences we have described. Specimens 

 from the upper bay have been taken principally from the region of Carquinez Strait, 

 those representing the middle bay from Goat Island and from the Southern Pacific 

 and Western Pacific moles, and those considered typical of the lower bay have been 

 collected at Dumbarton. 



Variation in Number of Ridges 



It was obser\-ed in the course of a study of the rate of growth of the shell (see p. 

 289), that shells from Goat Island show a slight but consistent advantage in number 

 of ridges over shells of the same age from Crockett. The significance of this was not 

 immediately evident. In order to let further light on the matter, it was decided to 



