58 THE ORIGIN OF VERTEBRATES 
by means of which blood should be brought to the surface, so as to 
interchange its gases with those of the external medium; and it is 
significant to find that of all vertebrates the Amphibia alone are 
capable of an effective respiration by means of the skin. 
As to the circulatory system, it is exceedingly easily modified. 
An animal such as Amphioxus has no heart; in some the heart is 
systemic, in others branchial ; in some there are more than one heart ; 
in others there are contractile veins in addition to a heart. There 
is no difficulty here in altering and modifying the system according 
to the needs of the individual. 
For a digestive system all that is required is an arrangement for 
the digestion and absorption of food, a mechanism which can arise 
easily if some of the cells of the skin possess digestive power. Now 
Miss Alcock has shown that some of the surface-cells of crustaceans 
secrete a fluid which possesses digestive powers, and she has also 
shown that certain of the cells in the skin of Ammoccetes possess 
digestive power. 
The difficulty, then, of forming a new digestive system in the 
passage from the arthropod to the vertebrate is very much the same 
as the difficulty in forming a new respiratory system in the passage 
from the water-breathing fish to the air-breathing amphibian—a 
change which does not strike us as inconceivable, because we know it 
has taken place. 
The whole argument so far leads to the conclusion that vertebrates 
arose from ancient forms of arthropods by the formation of a new 
alimentary canal, and the enclosure of the old canal by the growing 
central nervous system. If this conclusion is true, then it follows 
that we possess a well-defined starting-point from which to compare 
the separate organs of the arthropod with those of the vertebrate, 
and if, in consequence of such working hypothesis, each organ of the 
arthropod is found in the vertebrate in a corresponding position and 
of similar structure, then the truth of the starting-point is proved as 
fully as can possibly be expected by deductive methods. It is, in 
fact, this method of comparative anatomy which has proved the 
descent of man from the ape, the frog from the fish, etc. 
Let us, then, compare all the organs of such a low vertebrate as 
Ammocecetes with those of an arthropod of the ancient type. 
