No. 3-J VARIATION IN THE TEETH OF NEREIS. 137 



delicate jaws were often finely denticulated to the extremity. 

 The serratures of the older jaws appeared to be worn off by 

 use. In order to determine whether or no the correlation 

 actually existed between the size or age of an animal and the 

 number of definite teeth, I made the comparison for one hun- 

 dred individuals, taking as an indication of size, and hence 

 roughly of age, the head length in millimeters. This was meas- 

 ured from the anterior edge of the first ring to the extremities 

 of the two apical feelers. The result was a negative correlation, 

 although a rather small one, 0.128. It is probable, then, that 

 age does come in as a factor in the production of a small number 

 of teeth, and that this decrease is due to wear. It is possible 

 also that the wear may be heavier upon one jaw than upon the 

 other, thus accounting for the comparatively slightly lower 

 degree of correlation between the definite teeth than between 

 the indefinite teeth. For wearing does not act at all directly 

 on the indefinite teeth, since they do not emerge from the 

 chitinous covering, and are often sunk in the tissue of the pro- 

 boscis. It would be interesting to know in what manner the 

 jaws are carried and work upon each other during the life of 

 the animal, for a certain habit of crossing them might also 

 account for the peculiar differences in skewness of the curves 

 of the right and left teeth noted in the discussion of con- 

 stants. The smallness of the negative index of correlation 

 between the age of an animal and the number of definite teeth 

 shows that age does not play a very important part in causing 

 variation. 



An attempt to correlate the number of definite and the num- 

 ber of indefinite teeth on the right jaw resulted in a negative 

 index of correlation, 0.424. This fact indicates an inverse 

 relation between the definite and indefinite teeth on the same 

 jaw; that is, a jaw with a small number of definite teeth will 

 probably have a comparatively large number of indefinite teeth, 

 and inversely. It may be that indefinite teeth continue to be 

 laid down at the base of the jaw during the life of the animal, 

 in which case the number would tend to be greater with age, 

 while, as we have seen, the number of definite teeth is 

 smaller. 



