Figure 7

Comparative morphology of the hagfish. A: Differences in anatomical patterns between the hagfish and other vertebrates. Generally in vertebrates, hypoglossal or occipitospinal nerves (XII) pass along the posterior edge of the pharynx, and when the accessory nerve (XI) is present, nerve XII passes medial to nerve XI and lateral to the vagus nerve (X) to innervate the hypobranchial muscles (hbm). Thus nerve XII does not pass within the lateral body wall. In the hagfishes, putative hypobranchial muscle is assumed to arise in the rostral part of the ventral muscle that continues posteriorly into the rectus muscle in the trunk. Here, nerve XII does not form a single nerve trunk, but segmental occipitospinal nerves are shifted rostrally and no longer circumvent the pharynx caudally. However, this nerve still lies in the neck lateral to nerve X, as seen in the lampreys and gnathostomes. This peculiar morphology in the hagfish is thought to be due to a secondary modification of embryonic development, which is regarded as an autapomorphy for the hagfish. B: Homology of the hypobranchial muscle in the hagfish. The ventral somitic muscles of the hagfish can be seen as ventrally overgrown Lbx1-positive somitic muscles (dark green) in the larval lamprey. In the hagfish scheme, the pars decussata on the contralateral side is flipped back to the original (left) side of the body, and suspended ventrally. On the basis of this similarity, the homologue of the hypobranchial muscle is identified in the rostral oblique muscle with pars decussata in the hagfish.