CHILD. 



[VOL. I. 



being mere wrinkles on the surface. The ventral furrow can be 

 traced to the anterior boundary of c, where it ends very near the 

 edge, while the dorsal furrow ends free about midway of the 

 segment. Thus the segment c, as bounded by the very shallow 

 curved furrows, does not reach the right edge of the body at all 

 on the ventral surface, and dorsally extends to the edge only 

 anterior to the end of the curved furrow. The portion forming 

 the edge is not separated from b. At the left the ventral furrow 



between b and c reaches 

 the edge normally, but 

 ends there, for the dorsal 

 furrow, instead of extend- 

 ing to the edge and meet- 

 ing the ventral, bends for- 

 ward like the right ends 

 of the furrows and be- 

 comes like them very 

 shallow, a mere wrinkle, 

 which is visible to the 

 anterior end of the seg- 

 ment. Thus the portion 

 forming the left edge in 

 the region of c is con- 

 nected on the ventral sur- 

 face with c, but dorsally 

 only with b, i.e., there is 

 a short spiral here. This 

 case shows how a spiral may arise by the bending forward of 

 one of the furrows to meet the next in front, instead of uniting 

 with its fellow on the opposite surface. In nearly every case of 

 spiral variation this bend occurs as it does here near the edge, 

 though in many cases the furrow remains of normal depth. This 

 case then is what may be called an incipient spiral modification. 



The spiral does not appear between c and b at the right, simply 

 because both furrows bend forward, though the furrow does not 

 completely separate the two segments, but ends free. The fur- 

 rows anterior to c must be considered briefly before turning to 

 the discussion of the genital organs of b and c. The dorsal 



FIG. 20. 



