Figure 1

The morphology of H. polygyrus and H. neopolygyrus. A) a cross section through the mid-region of H. polygyrus female; B) a cross section through the mid-region of H. neopolygyrus female; In both cases, the worm is oriented with the head away from the viewer, with the dorsal surface at the top of the image. Slender black arrows indicate the ventral crête of the respective worms; the short black arrows indicate the ventral left-hand quadrant where the crête is largest in H. polygyrus but not in H. neopolygyrus. C) bursa of H. neopolygyrus; D) bursa of H. polygyrus; White arrows indicate the externo-dorsal rays, showing the enlarged base in H. polygyrus but the filiform ray present in H. neopolygyrus.