BIHANG TILL K. SV. YET.-AKAD. HANDL. BAND '27. AFD. IV. N:0 4. 11 



jiiv. < sen. 



Vahlii-grou]) < retictilatus-gron-p', 

 and tlie specific distinctness may be acknowledged, although, 

 as \ve have seen, of very sliglit valne; the one group having 

 to be regarded as a farther advanced stage of the common 

 development, witli the mascnline characters as leading tbe 

 advancement. 



All tbese differences in the position of the anal opening. 

 we have seen, are depending on the length of the head; but 

 there is of course another factor also determining that posi- 

 tion, viz. the length of the trunk (abdomen, tr in our tables 

 of measurements). A comparison of this length with the 

 head gives the following result: 



Åverages 



gracilis 



palUdus 



Vohlii frigiäus 



semt- 

 nudus 



reticn- 

 latus 



a . . . 



106.3 

 114.1 



155.5 76.3 

 113.2 94.5 



114.6 97.1 

 97.6 87.9 



182.51120.9,261.5 

 91.1 90.7 97.5 



139.8 235.7j 96.4|161.3 

 92.61 94.5 95.7 92.0 



or 



Here we see that the above cited anxological law is broken, 

 for in each group (excepting the two extremes gracilis and 

 reticulatus) the respective forms have the per-cent-niimbers 

 of the young less than those of the older age (juv. < sen.), 

 but the form-series of the Fa^Z^^-group goes from the higher 

 to the lower figures {gracilis > pallidus > Vahlii), and in the 

 rcticiilatus-gron]) the form-differentiation is almost none. In 

 the first place this is a natural consequence of the sexual 

 differentiation ; for the females, as we have seen in all other 

 respects, normally represent the juvenile stages, but here, 

 for the sh orter head, present higher per-cent-numbers, thus 

 in averaffe: 



