GILL CLERIS AND HEART 
though dissociated from the respiratory function, 
remains for life. That cleft is the external ear hole, 
which opens internally by what is called the eustachian 
tube into the pharynx. Thus, though the gill clefts in 
the higher vertebrates are no longer used for breathing 
purposes, being replaced by the lungs, they persist fora 
time to emphasize the relationship between the higher 
and the lower vertebrates. 
The ventrally placed heart is another marked 
characteristic of the vertebrata. The beat of the heart 
in man is felt through the chest walls and not through 
the back. In all animals the heart has the same posi- 
tion ; it lies on the opposite side of the body to that which 
lodges the brain and spinal cord. In many invertebrate 
_ animals the position of these two important organs is 
exactly the reverse. Inworms, insects and crustaceans, 
the heart, if it exists, is on the dorsal side and the 
larger portion at any rate of the central nervous system 
is on the ventral side, the side upon which the animal 
progresses, or which is turned towards the ground in 
progression as in those animals which possess legs. 
Anatomy reveals many other characteristics of 
vertebrate animals; but the principal ones are those 
which we have just briefly sketched. The large as- 
semblage of creatures classifiable as vertebrates splits 
up readily into the five sub-groups mentioned above, 
all of which are considered in the following pages. 
