Eaymond Pearl and Wilbur N. Fuller 
223 
That the posterior end of the cliteUum shows the least variation of any point 
in connection with this organ is further shown by direct measurement. The 
frequency distribution concerned is as follows : 
Number of Somites 
to posterior end of 
Clitellum 
Frequency 
36 
40 
37 
416 
38 
38 
39 
1 
Total 
495 
The mean deduced from this is 37 somites and the standard deviation '407, 
giving a coefficient of variation of I'lOO %, which is significantly lower than that 
for the position of the anterior end of the clitellum. 
This relatively fixed position of the posterior end of the clitellum is very 
probably associated with the fixed position of the openings of the male ducts and 
the other parts of the reproductive system in this form. The clitellum is to be 
regarded as an accessory organ, at least, of the reproductive system. It has been 
shown by teratological work that the various portions of the reproductive system 
proper vary, so far as position is concerned, as a nnit*. Whenever one part 
develops in another somite than that in which it normally occurs all the other 
parts appear in abnormal positions such that the normal relations between them 
are preserved. Beddardf has shown that in the highly variable form Perionyx the 
same sort of relationship holds between the position of the clitellum and the other 
organs of the reproductive system proper. 
We may now turn to the consideration of the exact form which the clitellum 
takes in different cases. It was found during the collection of the data, that the 
clitellum did not, in all cases, completely include all the somites which it touched. 
In other words, it was rather frequently observed that clitellar tissue would only 
partially cover a somite at the anterior or the posterior end of the organ, or, in 
rare cases at both ends. It is of interest to know how frequently this condition 
occurs, and accordingly we have prepared a table to bring out this point. The 
following method was used in recording clitellum form : in case the clitellar tissue 
ended 'at both extremities of the organ, precisely at intersegmental grooves, two 
parallel, unbroken vertical lines were made, with a figure indicating the number of 
somites in the clitellum between them. Thus | 6 | indicates that the clitellum 
* Cf. Bateson, Materials for the Study of Variation, pp. 159 — 167, and a paper by the present 
writer in the Anat. Anz. Bd. xviii. pp. 123 — 127 describing a case in which this close interconnection of 
the parts in respect to position is very clearly shown. 
t Proc. Zool. Soc. 1-886, pp. 298—314. 
