2OO PAUL S. WELCH. 



If the bifurcation plane is frontal in position each nerve chain 

 passes unchanged into each of the branches. Certain modifi- 

 cations of this arrangement occur if the plane of division be 

 sagittal, chief among which is the swinging to one side or the 

 other of each nerve cord. 



Nephridia are sufficiently developed in these forms to be 

 detected and one pair is normally related to each nerve cord in 

 each somite, that is, two pairs per somite in the trunk portion. 

 In the branches there is only one pair per somite. 



In the bifurcate portions, the main vessels of the circulatory 

 system occur in the normal position, but at the origin of the 

 branches, the two dorsal vessels approach each other and are 

 then laterally displaced by the union of the alimentary canals, 

 one occupying a position to the right, the other to the left. A 

 ventral vessel occurs just above each nerve cord. 



It thus appears that at least some of these double embryos 

 are of the type representing dorsal union, the type which Vej- 

 dovsky (1888-1892, p. 257) evidently referred to as "Die Dop- 

 pelmissbindung, deren Individuen langs der Riickenseiten 

 verwachsen," and which Weber (1917, p. 342) has also discussed. 

 While none of the other types found by these authors in double 

 embryos of certain Lumbricidae have thus far been detected in 

 Tubifex tnbifex it is possible that some or all of them will appear 

 when a more extensive study involving a much larger quantity 

 of material is completed. Such a study is in progress and results 

 will be reported in a subsequent paper. No attempt is made at 

 present to account for the origin of these anomalies. 



SUMMARY 



1. Tubifex tubifex (Mull.) deposits from one to seventeen eggs 

 per cocoon, a few of which are often sterile. 



2. Emergence of young worms from the cocoon is exclusively 

 through two oppositely located apertures. 



3. Various forms of abnormality commonly appear among the 

 developing worms, chief among which are bifurcations of the 

 body. Approximately twenty percent of the cocoons examined 

 contained bifurcate embryos. This is the first report of bifur- 



