VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 359 
walls of the coelomic cavity, and hence not from branchial 
myotomes. 
The development of these muscles in Ceratodus, as 
described by Greil, may perhaps offer an explanation of this 
difference of innervation and apparent derivation of these 
muscles in these two large groups of fishes. The coraco- 
branchiales of Ceratodus are said by that author to be 
developed from a ventral process of the posterior half of the 
second trunk myotome. From a similar ventral process of the 
entire third trunk myotome a large muscle is said to be de- 
veloped (1913, p. 1140) which acquires insertion on the hypo- 
hyal and ceratohyal and hence is evidently a musculus 
coracohyoideus, and from this muscle the coracomandibularis 
is differentiated. Similar ventral processes of the fourth and 
fifth trunk myotomes form the posterior portion of the 
hypobranchial muscles, these portions evidently representing 
the musculus coracoarcualis of the Selachii ; and these 
muscles are all innervated by branches of a nerve formed by 
the fusion of the nerves of the fourth and fifth trunk seg- 
ments (myotomes). Of these nerves Greil says (loc. c i t ., 
p. 1139) : “ Es besteht jedoch keine engere Zugeliorigkeit zn 
den betreffenden Segmenten, jeder Nerv versorgt auch den 
Myotomfortsatz des vorderen Segmentes, was sich schon 
daraus ergiebt, dass der dritte Segmentalnerv in der Hegel 
keinen ventralen Nerven an die hypobranchiale Musculatur 
abgiebt.” Here it is said that the coracobranchiales and the 
coracomandibularis + coracohyoideus are derived from ventral 
processes of adjoining segments of the trunk which differ 
only in that one of them becomes affiliated with the branchial 
arches and acquires innervation by the vagus, while the other 
retains its affiliation with the trunk myotomes and acquires 
innervation by the nerve of the next posterior trunk seg- 
ment. This change of innervation, based on embryological 
evidence alone, I am always inclined to doubt, but it is to be 
noted that if the process of the second trunk myotome had 
retained its primitive relations to the other trunk myotomes, 
instead of undergoing some sort of change because of its 
