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these, certain distant relations of the Vertebrates ; namely, the 

 Tanicates 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, in the Fishes, it 

 is clearly seen to be a continuation of the main body ; while the 

 fore and hind limbs (that is. the pectoral and ventral fins) 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 celom.e, 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. 

 Eunning 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 vertebra 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 tig. 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. Heloiv 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 tin 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, 



