Sept. 3, 1885] 
NATURE 
419 
as Appendicularia, have never resought the shore, and 
consequently have never degenerated to qualify for 
littoral life. The peculiar breathing apparatus adopted 
by the vertebrata occurs nowhere else in the animal 
kingdom except in the extraordinary worm-like Balano- 
glossus. The apparatus, as is well known, consists of a 
series of slits, opening from the exterior at the sides of the 
fore part of the body directly into the throat, the anterior 
part of the digestive tract. The water to be respired is 
taken in at the mouth and ejected through the gill slits. 
The late researches of Mr. W. Bateson, of Cambridge, 
have shown that Balanoglossus, besides breathing by 
gill slits, shows many other remarkable affinities, both in 
structure and development, with the vertebrata. Now, 
Balanoglossus, a shore-inhabiting form which lives 
buried in the sand, is developed from a most remarkable 
larva known as Tornaria, which is intermediate in form 
between a Trochosphere and astar-fish larva. It is quite 
possible that this extraordinary Jarva Tornaria may point 
to the former existence of a primitive pelagic ancestor 
common to the Annelids, Echinodermata and Vertebrata. 
Possibly the use of gill slits as a respiratory apparatus 
first arose in a shore-inhabiting ancestral form, such as 
Balanoglossus, and hence their presence at the anterior 
extremity of the body, that nearest to the surface when 
the animal is concealed in the sand. 
It appears not impossible that Amphioxus may once 
have possessed a larval stage somewhat resembling 
Tornaria, following on its gastrula stage, and has lost 
it just as one species of Balanoglossus has lost the 
Tornaria stage. The developmental history of only one 
species of Amphioxus is as yet known, and investigation 
of that of other species may yet reveal something of the 
kind suggested. 
The littoral zone not only became itself stocked with 
an immense variety of specially adapted inhabitants, but 
has given off colonists to the three other faunal regions. 
The entire terrestrial fauna has sprung from colonists 
contributed by the littoral zone. Every terrestrial verte- 
brate, every frog, reptile, bird, and mammal, bears in its 
early stages of development the gill slits still perforating 
its threat as in its aquatic ancestor. The tadpole still 
uses them when young for breathing, though they close 
up completely in the adult frog and in all the higher 
vertebrates before birth. In some of the tailed Amphibia, 
like the Axolotl, the breathing is by external gills and 
also by lungs which are modifications of the air-bladder of 
fish. In these the gill slits remain open, although they 
have no longer any respiratory function. It is amusing 
to watch tame Axolotls when fed in aquariums with large 
worms. They snap the prey down hurriedly and close 
their mouths, but usually in a moment or two their throat 
begins to twitch uncomfortably as if intensely tickled, 
and one end of the worm appears out of one of the gill 
slits, and the worm soon wriggles its way out again. 
Often the Axolotl catches it again by the free end before 
the other is completely out of the gill slit, and begins 
another attempt to swallow it, and the process is some- 
times repeated several times before actual deglutition is 
effected. The gill slits are evidently a considerable incon- 
venience to the Axolotl. The frog is much better off in 
having them closed, but man himself is not in a position 
entirely to despise the Axolotl: his lungs are derived 
from the same source originally, namely, modifications of 
the air-bladder of a remote aquatic ancestor, an inhabitant 
of the sea-coast, and they open into the throat just behind 
the tongue. In man there is a lid to close this opening 
and a contrivance to pull it under the tongue when 
swallowing takes place ; but every one knows the agony 
entailed by getting a crumb the wrong way—an accident 
very much akin to that of the Axolotl, and similarly 
entailed by the use of a single passage for two different 
purposes—feeding and respiration. At such moments of 
suffering the naturalist is inclined to turn traitor and 
long that he had been produced in accordance with the 
hypothesis of special creation rather than evolved under 
the laws of natural selection. The existing arrangement 
must not be regarded as of inevitable necessity. The 
vertebrates are the only animals which breathe through 
their mouths. All other animals have separate passages 
for respiration and feeding. The common snail has a 
separate breathing passage completely apart from its 
mouth, the land crab breathes by openings at the bases 
of its legs, the scorpion by openings on its abdomen, 
and the insect by numerous apertures on the sides of its 
body. All these animals cannot, like man, choke them- 
selves. 
Only the pentadactyle vertebrata have adapted them- 
selves completely for terrestrial respiration, but several 
fish have, by special modification of their gills, become 
able to remain out of water for almost indefinite periods. 
Most remarkable amongst these is Periophthalmus, one 
of the Gobiadz inhabiting mud flats on the sea-shore in 
Australia, Ceylon. Fiji, and other eastern tropical regions. 
It hops along the mud with the greatest agility and so 
fast that it is most difficult to capture, and even refuses 
to take to the water when driven to it, skipping along its 
surface,,and resting on projecting stones. It even climbs 
high up the mangrove trees and sits on the branches. 
All modes of air-breathing are derived by modification 
from aquatic breathing apparatus, except, perhaps, in the 
case of the air-breathing tracheata, the insects and their 
allies, in the ancestor of which, represented by Peripatus, 
the respiratory tubes or tracheze were probably first 
formed as modifications of skin glands. 
Littoral animals of most various kinds have taken from 
marine to terrestrial life no doubt by gradual adaptation, 
owing to exposure by the tides. Crustacea seem to have 
the greatest power of thus adapting themselves to aérial 
respiration by slight modification of their gill apparatus, 
so as to permit it to act as a lung. Nothing is more 
astonishing to the naturalist in tropical countries than to 
find large crabs amongst the vegetation far inland and 
high up mountains. But land crabs are not confined to 
the tropics; in Japan they may be met with walking 
across the high roads far inland, and 4000 feet above 
sea-level. One of the most remarkable instances is that 
of the cocoanut climbing crab, Bixgus Jatro, which has 
developed, as Prof. Semper has shown, a regular pair of 
lungs out of the walls of its gill cavities. The animal 
was originally a hermit crab, but got too large for any 
shell, and thus developed hard plates on the surface of 
its body for protection instead. Close allies, but of much 
smaller size, swarm in some Pacific islands. They 
always bear shells, and carry them with them when they 
climb the trees and bushes. I have caught hold of the 
shell of one of them as it clung to the top of a branch, 
thinking that it was a land-mollusk, and have been 
astonished by receiving a sharp nip from a pair of claws. 
The oldest-known air-breathing animals, so far as geo- 
logical evidence goes, are scorpions and insects. .\n 
ally of the cockroach and two scorpions have lately 
been obtained from Silurian strata. The close affinities 
of the scorpions with the king crabs, and thus with the 
Trilobites, is a most interesting matter, which has lately 
been urged by Prof. Ray Lankester. He suggests that 
the lungs, by means of which the scorpions breathe air, 
are modifications of the gill plates of the king crab, which 
have become inverted for the purpose. The lung open- 
ings of Scorpio correspond with the gill plates of Limulus 
in position and number. Hence, possibly, the scorpions, 
and with them the rest of the Arachnida, are sprung from 
ancestral allies of the king crab and the Eurypterids, 
having passed front a littoral to a terrestrial existence. 
It seems possible that birds were originally developed 
in connection with the sea-coast, and were fish-feeders. 
The tooth-bearing birds discovered by Prof. Marsh, such 
as Hesperornis and Ichthyornis, were marine aquatic 
