VISCERA!- ARCHES OF THE GNATHOSTOME FISHES. 309 
But, whatever Dohrn may have considered to have been 
developed from the distal portion of the ventral end of the 
myotome of each branchial arch, it is certain that the 
nmsculus interbranchialis was considered by him to be inter- 
calated, in the embryos described by him, between dorsal and 
ventral muscles, the dorsal one of which, alone, was the con- 
strictor superficialis. This is, however, said by him (loc. cit., 
p. 144) to be a secondary condition, the constrictor superficialis 
having certainly, in earlier phylogenetic stages, traversed the 
related branchial diaphragm along with the interbranchialis ; 
and it is here further said, in direct contradiction to the 
statement made on p. 113 of his work and above referred to, 
that the constrictor superficialis is in reality simply the distal 
portion of the interbranchialis. How it was, or when, the 
interbranchialis became separated from the portions of the 
myotome dorsal and ventral to it, or that it ever became so 
separated, is not said ; and as it is definitely said (loc. cit., 
p. 119) that none of the muscles of the arch are directly 
inserted on any of the branchial rays, the extrabrancliials 
expressly included, it is certain that these three parts of the 
myotome were, in the embryos examined by Dohrn, simply 
arbitrarily established regions of a single continuous muscle. 
In the identification, in embryos, of the several muscles 
above referred to, Dohrn makes frequent reference to Vetter’s 
descriptions of the muscles in the adult Selacliii, the evident 
inference being that, unless otherwise stated, the muscles 
described by himself, in embryos, were to be considered to be 
identified with the similarly named ones in Vetter’s descriptions 
of the adult. To this Dohrn makes one exception, the coraco- 
branchialis, which has been above referred to and explained. 
No limitation or qualification of any kind is made by him in 
his use of the term musculi interarcuales, and this term is 
used, where it applies to the muscles of a single arch, both 
in the plural (loc. cit., p. 113) and in the singular (loc. cit., 
p. 115). This muscle (or muscles) is said to extend from the 
pliaryngobranchial of an arch to the corresponding element of 
the next posterior arch, and also (“ resp.”) to the epibranchial 
