A STUDY OF METAMERISM. 411 



lu fig. 21, A, B, c, a somewhat similar spiral involves the 

 9th, 10th, and 11th raetameres, and also introduces an extra 

 half-metamere on the left side. The external openings of the 

 vasa deferentia were not found, but dissection showed that the 

 ovaries were present in consecutive segments, i.e. each in the 

 13th half-metaraere of its side. The explanation is the same 

 as in the last case. 



In fig. 22, A, B, c, we find a spiral belonging to category 

 IX (PI. 40, fig. ix). An extra half-segment is introduced. 

 Nevertheless the openings of the vasa deferentia seemed both 

 present on the same ring (metamere). This metamere is made 

 up of the 15th half-metamere of one side and the 16th half- 

 metamere of the other. In fig. 23, a, b, c, d, are shown two 

 modifications in the anterior end. The first of these is brought 

 about by an incomplete lateral union (c) of the line between 

 the 5th and 6th metameres. More posteriorly a spiral is intro- 

 duced that involves the 13th and 14th metameres. An extra 

 half-metamere, as shown in d, is introduced on the left side. 

 The external openings of the vasa deferentia were not seen. 

 Dissection showed that only one ovary was developed, and that 

 on the right side (or at least only one was found). This was 

 in its normal half-metamere. The seminal receptacles were in 

 their normal segments (9th and 10th), which showed that the 

 more anterior modification (failure of lines to meet) did not 

 aflPect subsequent metameres. 



In fig. 24, A, B, c, a short spiral involving the 5th and 6th 

 metameres is present, and introduces an extra half-metamere 

 on the left side. External openings of vasa deferentia were 

 not found. Dissection showed that the ovaries were present 

 in the same segment (13th of one side, 14th of other side). I 

 could not fully decide whether this case was due to a true in- 

 tercalation of a half-segment or to an overgrowth of the middle 

 line by the right half 6th metamere, due to an incomplete 

 dorsal growth of the left half 5th metamere. The latter inter- 

 pretation would be more in accordance with the presence of 

 the ovaries in the 13th — 14th metamere, which would then be 

 only the 13th metamere. Moreover, as this is the only case 



