106 Transactions of the Royal Society of South Africa. 
external gills which project from the gill septum into the surrounding liquid 
medium. The egg case is constructed so that the water can pass in at one 
end and out at the other, and, in the somewhat elaborate egg of Callorhynchus, 
there is a very perfect device for providing a current of water, as the long 
flat tail of the embryo projects into a narrow part of the egg case, and keeps 
up a constant undulatory movement. 
This same diminution of the supply of water may occur in post- embryonic 
life, and here we find similar devices adopted, both the simple increase of 
vascular surface and the development of external gills. 
Two simple cases of the first which do not seem to have been previously 
recorded may be mentioned. One is that of a species of G-oby (Trypauchen) 
found in masses of mud brought up by dredge off Delagoa Bay on one or two 
occasions. When alive it was of a bright red colour, which microscopical 
examination showed to be due to blood contained in a network of fine vessels 
spread over the whole body under the thin transparent skin. The other is 
that of the blind deep-sea fish (Barathronus), a specimen of which was 
brought up from a bottom of Globegerina ooze off the Cape. Another 
example is the highly vascularised skin of the clawed toad, Xenopus, which 
spends most of its life under water. 
In post-embryonic stages we may also have the development of external 
gills, under conditions unfavourable for an adequate supply of oxygen, as, for 
instance, in the muddy or semi-aquatic environment of the Dipnoi, in which 
external gills are developed. 
It is obvious that a semi-aquatic environment must have been passed 
through by the numerous vertebrates which now lead a terrestrial life, and 
these, in all probability, passed through an external gill stage. Further, it 
is not improbable that migration to the land conditions began as early as 
migration to the pelagic conditions or at the Amphioxus-Myxine stage. 
(In this connection it may be mentioned that the Cape Bellostoma is said to 
make occasional excursions from its aquatic element.) What more probable 
than that the primitive limbless vertebrates, equipped with external gills, 
should, as has been suggested, use these as organs for the support and locomo- 
tion of the body under its new conditions of life, and from these the 
pentadactyle limb be developed ? 
If the tetrapod limb has been evolved in this way from an external gill, 
developed in connection with the transition of the primitive aquatic 
vertebrate to terrestrial life, it can hardly be supposed at the same time 
that from this organ arose the paired limbs of fishes, developed as organs 
necessary in the transition to pelagic life, probably in association with the 
shortening and flattening of the body. 
There have been belated attempts among fishes to acquire the terrestrial 
habits. The most successful, however, such as that of the Periophthalmus or 
Anabas, have shown no approach to the acquisition of an organ of terrestrial 
