398 T. H. MORGAN. 



The variations of this category are formed by the lines 

 turning both anteriorly or both posteriorly, and on the right 

 or left side of the body. In reality the half-segment is simply 

 intercalated between two perfect segments ; and when we speak 

 of the lines turning either forward or backward we only imply 

 that the intercalated piece has encroached more on the one or 

 the other segment. 



Category iv, fig. IV, a, b, c. — Here three half-segments of 

 one side unite with one half-segment of the opposite side, so 

 that the right side appears to be '' split " by two incomplete 

 divisions into three half-segments. This may be called a 

 double compound raetamere. Variations arise owing to the 

 depth to which the incomplete lines (splits) pass towards the 

 middle line. A combination of this form (really only a half- 

 double compound) with others is shown in PI. 40, fig. XIV. 

 This same modification may be carried a step further, and in 

 one case I found four half-metameres of one side united to 

 one of the opposite. See PI. 42, fig. 47. 



Category v, fig. V, a, b, c. — Here on the upper side appear 

 two compound segments — one on the right side, the other 

 on the left. On the lower surface, b, two perfect metameres 

 are found. By tracing the division lines between these seg- 

 ments from the lower to the upper side we see the result is 

 brought about by the failure of these two septal lines to meet 

 above ; each turns somewhat to one side. The reconstruction 

 shows that a middle connective is left to unite the two seg- 

 ments above. This is brought about by the fusion above of 

 the middle of the right posterior segment with the left anterior. 



The variations are few. The incomplete meeting may take 

 place either above or below, and the lines turn anteriorly or 

 posteriorly. 



Category vi, fig. VI, a, b, c. — The arrangement found here 

 may be produced from fig. v by extending the free ends of the 

 segment lines till the one meets the segment in front, and the 

 other the one behind. A reconstruction of such a figure 

 gives the spiral shown in fig. VI, c. This spiral form terminates 



