STRUCTURE OF DOUBLE MONSTERS IN THE EARTHWORM. 345 



2. Those cases in which the union is latero-dorsal. Thus far 

 only one example has been found of this group, individual No. 

 70 shown in Fig. 10, Plate II. Figs, n and 12, Plate II., show 

 sections through the joined region which in this case extends 

 through three segments. The first is very much more anterior 

 than the second as is indicated by the presence of the cerebral 

 ganglia (bg) of which there is in this monster only one set. The 

 commissures extending from it can be seen in the same figure in 

 which is also present the beginning of one of the nerve cords (we). 

 A study of the next few sections shows the connection of the 

 brain to the two cords and also a connecting band between these 

 two structures. In this manner the single greatly elongated 

 alimentary tract, which in this case is the result of a union, is 

 completely surrounded by nerve tissue. Fig. 12 gives a section 

 through a more posterior region showing the relative positions 

 of the ventral cords. 



In no part of the joined portion can there be found less than 

 eight pairs of setae indicating that the union does not extend 

 beyond the sides of the individuals. 



The nephridee also are in their normal number and relationship 

 to the ventral cord. 



3. This is the group in which the union is end to end and in 

 which the cerebral ganglia are to be found on the opposite side of 

 the digestive tract from the ventral cord. It can be seen from an 

 inspection of Figs. 13, 14 and 15, external drawings of Nos. 90, 

 93 and 2, that the extent of union is limited to an unusually small 

 area. Sections show that the union is across the dorsal side, 

 for the ventral cords lie on the sides opposite that area, while 

 the cerebral ganglia lie on the same side. 



The digestive tracts have a common origin in each of the 

 three monsters arising from a single mouth opening from which 

 the pharynx passes into each individual. There is no noticeable 

 difference in the structure of any of the fused organs of these 

 forms as compared with those of No. 95 except in the nervous 

 system. 



A study of No. 90 will show a condition of that system some- 

 what different from any so far described. The ventral cord of 

 each individual lies on the side opposite the union while the 



