254 V. H. EDO E WORTH. 



in Amia, Salino, Esox, Menidia, may either retain its original 

 branchial innervation from tlie Xtb, e. g. Amia (Allis), 

 Esox (Vettei-), Menidia (Hei-rick), Lepidosteus, Polypterns 

 Senegal us, or be innervated by spino-occipital nerves, e.g. 

 Attiieurus (Wright), Salmo (Harrison). AVhen coraco- 

 branchiales are developed from all the branchial mj^otomes, 

 they are innervated by the spino-occipital nerves, e. g. 

 Selachii (Vetter, Fiirbringer), Acipenser (Vetter), Polypterns 

 ? species (Furbringer), Ceratodus (Fiirbringer). 



The coraco-branchiales muscles have been generally classed 

 with the hypobranchial spinal muscles, but investigation of 

 developmental stages shows that the ventral ends of 

 branchial myotomes ma}^ form longitudinal muscles, which 

 either grow forwards, forming interalrcuales ventrales, or 

 backwards, forming coraco-branchiales, but not in both 

 directions. (There are two, probably three, exceptions to 

 the above rule; in Amia, at a late stage of development, the 

 hind end of ihe interarcualis ventralis IV grows backward 

 to the lii'th l)ar; the innervation in Polypterns Sene- 

 gal us suggests that the hind end of the interarcualis ven- 

 tralis III similarly grows back to the fourth bar; and in 

 Ceratodus the hind end of the interarcualis ventralis I, at a 

 late stage of development, grows back, forming the coraco- 

 branchialis I, and also, at least in some cases, to a more 

 posterior bar.) The first condition, that of interarcuales 

 ventrales, is the primary one, as shown b}^ the correspondence 

 of cranial nerve innervation, with segment of origin. The 

 second condition, tliat of coraco-branchiales, is a secondary 

 cue, in which a change of function to one very similar to 

 that of the coraco-hyoideus is con-elated, though in varying 

 degree, with a change of innervation to one by the spino- 

 occipital nerves. 



An approximation to what was, probably, the primitive 

 condition, is seen in Amphibia. This was a series of 

 interarcuales ventrales, each extending from the bar of its 

 segment of origin to the next anterior one. The hyo- 

 maxillaris, in the hyoid segment, is seriall}^ homologous with 



