92 KARM NARAYAN BAHL 



scattorpcl arrangement of the integumentary nephridia (Text- 

 fig. 8). By comparing the position of the septal and integu- 

 mentary nephridia in Text-figs, 5 and 6 the discrepancy does 

 not seem to come to very much. But that there is a shifting 

 dorsalwards, however httle (and in many septa a great deal), 

 admits of no donht. This can he explained, however, easily, 

 if we take into account the facts of development and growth of 

 the septa and the structures connected with them, e.g. the 

 commissural vessel and the dorsal vessel. In the series of 

 diagrams in Text-fig. 8 I have tried to illustrate the gradual 

 growth and formation of the commissural which connects 

 the subn(nu'al and dorsal vessels. The dorsal vessel is formed 

 l)_y llic i)r()gressive backward concrescence of the two lateral 

 vessels, which, as a rule, lie at the dorsal edge of the advancing 

 mesoderm bands. The commissural, which appears very early 

 and connects the suhneural with the lateral (the semi-dorsal 

 so to speak), lies from the very beginning between the two sheets 

 of flic mesodermal septa. From an examination of figs. 13 

 and 14 it l)ecomes clear that the ' funnel-cell ' or the rudiment 

 of the septal nephridium is closely associated with the com- 

 missural vessel, lying in the same intraseptal cavitj^ with and 

 ventral to the l)lood-vessel. It would seem that in the process 

 of migration of the lateral vessels to the mid-dorsal position 

 the commissural vessel must keep pace and travel dorsalwards. 

 Furtlun-, in th(^ general growth of this vessel dorsally, the 

 ' funnel-cell ' or nephridial rudiment closely associated with it 

 also travels dorsalwards, and hence results the dorso-lateral 

 position of the primary septal nephridium. 



That the commissural vessel and the rudiment of the septal 

 nephridia lie in the same hollow of the septum between the 

 two sheets of mesoderm in the embryo is clear from figs. 13 

 and 14, and is evidence of the close association of these two 

 structures. That there is some morphological relationship is 

 also shown by the fact that, in the adult worm, the septal 

 nephridia are only present on those septa that have a com- 

 missural vessel. In the first fourteen segments of the worm 

 there are no septal nephridia and no commissm'al vessels either. 



