THE CHORDATA 135 



CHAPTER XIII 



THE CHORDATA 



THE older zoologists used to speak of Verte- 

 brata and Invertebrata as animals with a back- 

 bone and animals without one, and everyone 

 thought it a very natural way of dividing up the 

 animal kingdom. It never occurred to anyone 

 that it was possible to bridge the interval between 

 them and find a link between the two. But now 

 the Vertebrata have been compelled to give up 

 their aristocratic pretensions, and own that they 

 have risen from the ranks of the common people 

 of the animal world ; in other words, that they 

 are descended from the Invertebrates. Their 

 family secrets have been published to the world, 

 and now everybody knows that they have poor 

 relations. But how many, and how nearly 

 related ? This we do not accurately know ; 

 consequently the whole zoological world for 

 many years has concentrated all its energies 

 on attempts to find out the truth about the 

 matter. 



A great sensation was caused by the first dis- 

 covery of a poor relation of the vertebrates 

 among the Ascidians, or Leather-bottle animals. 

 These are named from their shape and texture, 

 for they have a leathery skin. Now some of 

 these Ascidians have larvae with a tail ; and in 

 the tail there is a long cord-like structure, which 

 in many essential particulars resembles the cord 

 which precedes the back-bone in the vertebrate 

 embryo. This structure is called the Notochord 

 (a string down the back). The credit of this 



