426 T. H. MORGAN. 



The septum is attached on the somatic side to the body-wall 

 along the " split." Centrally it is attached to one half of the 

 splanchnic wall of the digestive tract. We should find on 

 sectioning such a worm that the half-septum had a double 

 wall, and was like a true half of a septum in every respect. We 

 should see also, as a result of this arrangement, that the body 

 cavity of the single half-metamere is continuous in the mid- 

 ventral and mid-dorsal lines with two half-metameres on the 

 opposite side. 



The number of the nephridia corresponds to the number of 

 the half-metameres, as shown in fig. 63, so that there is one 

 more nephridium on one side than on the other. 



The nervous system is often modified, as shown in fig. 62. 

 Here each of the half-metameres is seen to receive its full 

 number of nerves from the ventral cord. This is not always 

 the case, for, as shown in fig. 61, b, the double side gets only 

 the supply normal for a single half-metamere. In figs. 63 

 and 64, b, we see other irregularities in the nerve-supply. 

 This may be connected with the degree to which the half- 

 septum approaches to the mid-ventral line. When it falls far 

 short the nerve-supply is less than when it reaches the mid- 

 line. The nephridia, however, that lie laterally in the body are 

 invariably doubled. In fig. 64, a, the surface line between the 

 half-metameres of the compound metameres is not very 

 extensive. The figure represents the body-wall flattened out 

 (the worm had been previously opened). Fig. 64, b, shows 

 that the septum s is attached to the somatic wall over a 

 correspondingly small area. 



The condition of the septa in the spiral modification is very 

 interesting. One of the simplest cases is shown in fig. 65, a,b. 

 The first figure shows a short spiral beginning and ending 

 above (category viii). On opening the worm from below 

 (fig. 65, b) the septa are seen to follow the same arrangement, 

 both on the body-walls and on the intestine. There results a 

 single spirally winding septum beginning with the 

 half-septum of the compound raetamere anteriorly, 

 and ending with another compound metamere poste- 



