Figure 8

The metameric pattern of H. dujardini includes five distinct segment identities. (A) A model H. dujardini segmental anatomy based on results of the investigations of ventral nervous and muscle systems presented here. Dashed lines demarcate the hypothesized boundaries between segments. Nervous system structures are depicted in red. Muscle system structures are depicted in green. Ventral attachment sites are numbered. Dashed lines represent muscles that extend dorsally. (B) Hypothesized anterior Hox gene expression domains of the onychophoran-arthropod ancestor based on [63]. The identities of onychophoran segments are provided on the left and the identities of insect segments are provided on the right. Dashed expression domains represent uncertainty in reconstruction of ancestral anterior expression boundaries. The anterior-most expression boundary of the Hox genes is the anterior boundary of the third segment. This pattern in H. dujardini would support direct alignment of tardigrade segments with arthropod and onychophoran segments and would suggest that the tardigrade head is composed of a single segment. Abbreviations: Dfd, Deformed; ftz, fushi tarazu; ga, ganglion; lab, labial; il, inner lobe; mo, mouth; ol, outer lobe; ph, pharyngeal bulb; pb, proboscipedia, Scr, Sex combs reduced; vl, ventral lobe.