16 
these, certain distant relations of the Vertebrates; namely, the 
Tunicates or Ascidians. 
Our first step must be a definition. What do we understand by 
the tail of a Vertebrate? If we look at one of the higher, forms, 
such as a pig or a rat, we may be apt to regard the tail as merely 
an appendage to the hinder part of the body; the animal is com- 
pletely organized, and capable of living in health, without it. But 
as we descend in our: survey to lower forms, we find the tail 
assume a more and more important position; till, im the Fishes, it 
is clearly seen to be a continuation of the main body; while the 
fore and hind limbs (that 1s, the pectoral and ventral jins) appear, 
by comparison, quite insignificant appendages. 
The vertebrate tail, then, may be defined as a prolongation of 
the main axis of the body, extending behind the great body-cavity 
or celome, and the digestive canal which lies therein. 
As a typical form, we may take the full-grown tail of the common 
tadpole, before it begins to shrink. fig. 1 is a side view of this. 
Running through its whole length, m the middle, we find a 
tapering, flexible rod, around which the other parts are arranged. 
This is the hindmost part of the notochord. This notochord is one 
of the chief marks of a vertebrate animal; all of them have it, at 
least in their young state. It extends from the region of the mouth 
backwards ; and forms the axis on which the bony vertebre of the 
spine are built up. In this tadpole’s tail, however, there are no 
bones at all. 
Just above the notochord lies the main nerve of the tail ; as shown 
in Fig. 2, which is a transverse section of the whole tail. This 
nerve is a prolongation of the great nervous axis that forms the 
brain and spinal cord. Below the notochord runs an artery, the 
continuation of the dorsal aorta, which brings pure blood from the 
gills. Beneath this again is a vein, which returns the blood 
towards the heart. In a young tadpole there are no other blood- 
vessels in the tail but these. 
These four central organs are surrounded by muscles, which are 
arranged in a series of distinct segments, from the base of the tail 
to near its tip. Lastly, we find a thin, transparent membrane 
extending in the middle line all round the tail, and even a little 
further, along the back and front of the body itself. This is the 
simplest form of a yin for swimming in water. The whole tail is 
covered with a soft, naked skin. We shall find that all the won- 
derfully varied forms of tails, in all classes of Vertebrates, may be 
derived from the primitive form seen in the tadpole. 
The most antiquated form of Vertebrate yet. discovered is the 
Lancelet (Amphioxus), which is living on the sea-bottom near our 
own shores. This little creature cannot well be reckoned a fish, 
