318 
HENRY ORB. 
properly be assigned to the hyoid arch. Balfour, iu describing 
the embryology of the Urodela(‘ Comp. Embry.’), says,“ Stalked 
suckers of the same nature as the suckers of Anura are formed 
on the ventral surface behind the mouth.” The balancers in 
the two forms of Urodela which I have examined possess none 
of the characteristics of suctorial organs. Yet the balaucers of 
the Urodela and the suckers of the Anura serve ultimately the 
same purpose, namely, to prevent the embryos from sinking into 
the soft organic mud usually found iu the bottom of the pools 
which they inhabit, immersion iu which would undoubtedly 
prove fatal to large numbers. An examination of a larger 
number of species may bring to light iutermediate forms of 
these organs which would prove a more direct homology 
between the balancers and suckers. 
The condition of the branchial apparatus and the skeleton 
of the head at a time when the branchial clefts have opened 
and shortly after the cartilage has appeared, is shown in figs. 
27 h — 31, and also fig. 18. Fig. 31 is an approximate recon- 
struction from drawings of sections in three planes at right 
angles to each other; it represents the skeleton of the left side 
of the head. The four branchial clefts (II — V) are situated 
between the hyoid arch (I') and the posterior branchial arch 
( V', fig. 30 h). Each of the posterior four cartilaginous arches 
(II — V, fig. 28 h) supports an external gill. The carti- 
laginous hyoid arch has no external gill, but supports an 
opercular fold (o./., fig. 30 h) which extends transversely across 
the ventral side of the head (fig. 18, o.f.) and a short distance 
up the lateral sides, partly overlapping the external gills. The 
cartilaginous bars of the visceral skeleton are of unequal 
length. Only the hyoid and the first two branchial bars 
extend to the median line, where they unite in a basi-hyo- 
branchial plate of cartilage ( B . Hy., figs. 31 and 18). From 
this basi-hyobranchial plate there extends in a ventral and 
posterior direction a long curved process of which the flattened 
end touches the pericardium. The posterior two branchial 
bars (IV , V') each unite with the next preceding bar as 
shown in fig. 31. The hyoid bar does not extend dorsalwards 
