In Actinopterygians and at least some Selachians a _lateralis_ set of fibres is associated with IX., and in the former fishes a conspicuous trunk of _lateralis_ fibres passes to some or all (_Gadus_) of the fins. This has been called the _lateralis accessorius_ and is apparently connected with V., VII., IX., X. and certain spinal nerves.[53] _Vagus Group_ (IX., X., XI.).--The _glossopharyngeus_ (IX.) forks over the first branchial cleft (pretrematic and post-trematic branches) and also gives off a palatine branch (pn.IX.). In some cases (various Selachians, Ganoids and Teleosts) it would seem that IX. includes a few fibres of the _lateralis_ group. Vagus (X.) is shown by its multiple roots arising from the medulla and also by the character of its peripheral distribution to be a compound structure formed by the fusion of a number of originally distinct nerves. It consists of (1) a number of branchial branches (X.¹ X.² &c.), one of which forks over each gill cleft behind the hyobranchial and which may (Selachians) arise by separate roots from the medulla; (2) an intestinal branch (v.n.X.) arising behind the last branchial and innervating the wall of the oesophagus and stomach and it may be even the intestine throughout the greater part of its length (_Myxine_). The _accessorius_ (XI.) is not in fishes separated as a distinct nerve from the vagus. With increased development of the brain its hinder portion, giving rise to the vagus system, has apparently come to encroach on the anterior portion of the spinal cord, with the result that a number of spinal nerves have become reduced to a less or more vestigial condition. The dorsal roots of these nerves disappear entirely in the adult, but the ventral roots persist and are to be seen arising ventrally to the vagus roots. They supply certain muscles of the pectoral fins and of the visceral arches and are known as spino-occipital nerves.[54] These nerves are divisible into an anterior more ancient set--the occipital nerves--and a posterior set of more recent origin--(occipito-spinal nerves). In Selachians 1-5 pairs of occipital nerves alone are recognizable: in Dipnoans 2-3 pairs of occipital and 2-3 pairs of occipito-spinal: in Ganoids 1-2 pairs occipital and 1-5 pairs occipito-spinal; in Teleosts finally the occipital nerves have entirely disappeared while there are 2 pairs of occipito-spinal. In Cyclostomes no special spino-occipital nerves have been described. The fibres corresponding with those of the _Hypoglossus_ (XII.) of higher vertebrates spring from the anterior spinal nerves, which are here, as indeed in Amphibia, still free from the cranium.