62 THE GENEALOGY OF THE VERTEBRATA. 



Accepting this phylogeny, it becomes possible to de- 

 termine the course of development, first, of the whole 

 series, and secondly of the contents of each class taken 

 by itself. 



I. THE LINE OF THE UROCHORDA. 



Embryological evidence leads ns to anticipate that the 

 primitive Vertebrate possessed nothing representative of 

 the vertebrate skeleton beyond a chorda dorsalis. Above 

 this axis should lie the nervous chord, and below it the 

 nutritive and reproductive systems and their appendages. 

 Such a type we have in its simplest form in the Branchi- 

 ostoma, the representative of the division of the Acrania, 

 In the animals of this division the mouth and anus have 

 the usual vertebrate position, at opposite ends of the 

 body-cavity. The Tunicata (formerly referred to the 

 Mollusca) are now known to present a still more primi- 

 tive type of Vertebrata, to which the name of Urochorda 

 has been given. These curious, frequently sessile crea- 

 tures, have a vertebrate structure during the larval stage, 

 which they ultimately lose. They have the necessary 

 chorda, and nervous axis with a brain, and a cerebral 

 eye. They have at this time a tail, and are free-swim- 

 ming ; a peculiarity which few of them retain throughout 

 life (Arrpendicularia). 1 They differ from the Acrania in 

 the positions of the extremities of the alimentary canal. 

 The mouth is on the top of the anterior end of the animal, 

 and is supposed by some anatomists to represent an open 

 extremity of the pineal gland of other Vertebrata ; while 

 the tract represented by this body, the third ventricle of 

 the brain, and pituitary body of the Craniata, are there 

 mains of the primitive oesophagus of the Urochorda. 

 The anus in the adult tunicates is either dorsal, or it 

 opens into the body cavity, as in the young larvae. In 

 Appendicularia it is ventral (Gegenbaur). 



1 Sec Lankester on Degeneration, Nature Series, 1880. 



4=6 



