480 RICHARD ASSHETON. 
This system might very well also be a comparatively recent 
adaptation; but, on the other hand, its very early development 
in the embryo, and its appearance in connection with the edges 
of the neural epithelium before there is any sign of gill-fila- 
ments, suggest a much more archaic origin. 
The third system, that which produces a flowing into the 
stomodzal pit, has certainly no obvious use to the embryo, 
although, in a small free-swimming form with open mouth and 
complete alimentary canal, such a current of water would have 
had an important function. 
Although I do not lose sight of the possibility of the whole 
ciliation of the amphibian tadpole being ccenogenetic, yet the 
occurrence of ciliated tracts, which may be compared in posi- 
tion and relation to such important morphological features as 
the blastopore and mouth, with certain ciliated tracts of Tor- 
naria and Echinoderm larve is, at any rate, worthy of notice. 
A ciliated ectoderm has never been described, as far as I know, 
for any Elasmobranch. I have examined living specimens of 
one member of this group, Scyllium canicula, at various 
ages during the first four months of development, and have 
never found any trace of ciliation. Although presumably a 
more ancient type than the Amphibians, the great difference in 
the condition of the egg and surrounding fluids may be suffi- 
cient to account for the disappearance of a primitive ciliation ; 
for of what use could a ciliation be immersed in the very 
viscid, almost jelly-like albuminous fluid surrounding the ovum 
in an Elasmobranch egg-capsule ? 
It is interesting to note that in default of a ciliation to pro- 
duce a constant flow of water over the gill-filaments and skin, 
the Elasmobranch embryo maintains an incessant undulating 
movement of its body from the time it is sufficiently folded off 
from the yolk until the time of hatching. 
Similar conditions at first present the same objection to the 
occurrence of a ciliated ectoderm in the Amniota, in whose case 
a special organ of respiration is subsequently developed. 
Tn none of these cases is it surprising to find a ciliation absent. 
In the case, however, of embryos developing from holoblastic 
