498 Development of Mouth and Gills in Bdellostoma 
they are in fact the forward fleshy extensions. Fig. 14 is a section 
further back in which the two forward parts have fused medianly to form 
the dental-plate, and on this bilateral body the teeth are developing. 
These now appear upon its dorsal surface ‘instead of on the two 
median surfaces as in Fig. 11, for the median surfaces which faced one 
another when the teeth began to form, now spread apart and become 
the final upper or dorsal surface of the dental-plate. The thin cartilage 
of the plate is seen below the dental surface. 
From the foregoing account a critic might object that, if we interpret 
the tongue as the lower jaw, we have a jaw of a most unorthodox 
type, for it develops in such a way that one portion of it may be 
moved out between its two other parts; a condition which seems at 
first sight mechanically impossible. But it must be remembered that 
the forward attached parts are only the fleshy folds of the jaw, mere lip- 
like structures, while the dental-plate itself has a sheet-like membrane 
spreading out from its four sides and loosely folded about its edges. 
This membrane is continuous with the lining membrane of the mouth 
cavity. By means of the highly modified muscle system the tooth-plate 
may be lifted up and carried forward, owing to its loose-folded mem- 
branous connection. ‘The movements, moreover, in the living animal 
have recently been explained by Worthington, 05. 
It follows finally from the discussion of the development of the mandi- 
bular arch and dental-plate in Bdellostoma that the organ long called 
the tongue in myxinoids is in no sense comparable or homologous with 
the vertebrate tongue. In this case, therefore, it should no longer bear 
so misleading a name, since the word tongue must necessarily imply 
or suggest a tongue-like structure. The term dental-plate which has 
been applied by several writers to this organ seems a very appropriate 
one. It indicates the nature of the organ and in no way interferes with 
the understanding that it is actually homologous with, though super- 
ficially widely different from, the lower jaw of all other gnathostomes. 
The Marsipobranchii have been so repeatedly placed among the Gnatho- 
stomata that the group will scarcely seem new in its correct position. 
THE DEVELOPMENT AND FATE OF THE HYOMANDIBULAR GILL CLEFT. 
This gill cleft in the youngest stages is strikingly similar to the 
mandibular, both having extensively developed diverticula bending up- 
ward in a manner typical for all of the gills. Dean has stated, p. 270, 
thatthe mouth is distinctly paired in character, but unlike the hyomandi- 
bular pouches, its diverticula are shown in section to bend downward, 
