Raymond Pearl and Wilbur N. Fuller 
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worms from this same locality, and secondly because such an explanation would 
not in any way account for the smaller niunhev of somites. 
Altogether, it must be concluded that the typical form of Lumhricns of this 
locality is very considerably smaller and contains fewer somites than the type of 
the species to which it is most closely related ; viz. L. herculeus*. There is no 
evidence available as to what is the cause of this dwai-fing, but it is not unlikely 
that it is due to direct environmental influence. 
5. Summary. 
A study of variation and correlation in several characters of the earthworm 
Lumhricus herculeus, Sav., leads to the following conclusions : 
1. The earthworm is more variable in length of body than in number of 
somites. 
2. There is a rather low degree of correlation (?'="260) between these two 
characters. 
3. The results just stated (1 and 2) arise from the fact that the organism 
increases in length as a consequence of the combined action of two processes, 
(a) the addition of new somites, and {h) the growth of somites already added. 
4. There is greater variability in the number of somites in the clitellum than 
in the number of somites lying in front of that organ. The factors concerned in 
this greater variability are analysed. 
5. The number of somites included by the clitellum is negatively correlated 
to a relatively high degree with the number of somites making up the portion of 
the body anterior to that organ. 
6. This implies that the posterior end of the clitellum is a relatively fixed 
point. This conclusion is confirmed by direct measurement. 
7. There is a strongly pronounced tendency for the clitellum to end both 
anteriorly and posteriorly exactly at intersegmental grooves. Overlapping occurs 
more frequently at the anterior than at the posterior end of the organ. 
8. The clitellum increases in extent as the worm grows in length. 
9. The form of Lumhricus found at Ann Arbor is shorter and contains fewer 
somites than the typical L. herculetis, Sav. 
In conclusion I wish to acknowledge my indebtedness to the officials of the 
Carnegie Institution for a grant to aid in the carrying on of certain biometrical 
work of which this paper forms a part. 
* There remains, of course, still another possibility, namely, that the values for the type of the 
species are based on a few unusually large specimens, but if this is the case it is difficult to understand 
why some one has not called attention to the matter. Especially does this seem remarkable when we 
remember that there is no organism more universally used as a type in zoological laboratories. 
