Charles R. Stockard 513 
evident from the foregoing facts that they are gnathostomatous verte- 
brates, and therefore the name cyclostomes should no longer be used 
in reference to them. The term Marsipobranchii proposed by Bona- 
parte in 1846 and adopted by Huxley, Parker, Beard, and others may 
very fitly be used to designate this group. This suggestion has been so 
frequently made in the literature of the subject that it sounds like a very 
old ery, nevertheless it is one that should be heeded as it will eliminate 
from our classification a term whose meaning is undoubtedly misleading, 
as well as one which will hinder the recognition of these animals as true 
jaw-bearing vertebrates. 
As to the position of the Marsipobranchii in the gnathostome series I 
have no definite opinion. These animals are, however, undoubtedly 
primitive as is indicated by the development of their mandibular arch 
and the presence of the additional anterior gill clefts along with other 
anatomical features such as a simple ear, straight digestive tract, ete. 
They are in other respects decidely specialized, as is indicated by the 
development of the dental-plate with its musculature, which is in reality 
the modified lower jaw apparatus. The peculiar pouched gills must also 
be regarded as specialized. The only point that I know of in the Bdellos- 
toma embryo to suggest degeneracy is the fact that a lens-like thickening 
of the ectoderm forms over the optic cup and later disappears. I should 
rather, however, interpret this as a case of arrested development since 
the adult eye is embryonic in its appearance, as was shown by Allen, 
05. Lewis, 04, has also shown that when the optic cup of the amphibian 
eye is removed, or does not continue its development, the lens will de- 
generate in the embryo. All of those features in Bdellostoma that have 
been accounted for as due to its parasitic or degenerate condition must 
be, in my opinion, explained on some other grounds. 
SUMMARY. 
1. In the development of the mandibular cleft of Bdellostoma the 
lateral diverticula at first turn dorsally, fusing with dorsal ectodermal 
plates in the same manner that all gills do. The diverticula later extend 
horizontally, and finally, bending ventrally, come to resemble the mouth 
arch of most other vertebrates. The fusion of the ectoderm and endo- 
derm continues through all the stages of development. 
These conditions suggest forcibly Dohrn’s idea of the origin of the 
vertebrate mouth from a pair of gill slits. 
2. The so-calied “tongue” of myxinoids is really the homologue of 
the gnathostome lower jaw. 
37 
