322 THE STRUCTUEAL EVIDENCE OF EVOLUTION. 



the scapular arch, the Mammalia have retrograded from the rep- 

 tilian standard as a whole. 



In subsequent pages I shall take up the lines of the classes 

 separately. 



III. THE LINE OF THE UKOCHOKDA. 



Embryological evidence leads us to anticipate that the primi- 

 tive Vertebrata 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 sys- 

 tems and their appendages. Such a type we have in its simplest 

 form in the Branchiostoma, the representative of the division of 

 the Acrania. In tlie 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 primitive type of Vertebrata, to 

 which the name of Urochorda has been given. These curious, 

 frequently sessile creatures, 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-swimming ; a peculiarity 

 which a few of them retain throughout life (Appendicularia).* 

 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 repre- 

 sent an open extremity of the pineal gland of other Vertebrata ; 

 while the tract represented by this body, the third ventricle of the 

 brain, and the pituitary body of the Craniata, are the remains 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). 



The history of the Tunicata can not be traced by paleontolo- 

 gists as yet, owing to the absence of hard parts in their structure. 

 The evidence of embryology has, however, convinced phylogenists 

 that the ancestors of this class resembled their larvae, and that they 

 have as a whole undergone a remarkable degeneracy. They have 

 passed from an active, free life to a sessile one, and have lost the 

 characters which pertain to the life of vertebrates generally. 



It was to have been anticipated, however, that all of these an- 



* See Lankester on "Degeneration," " Nature Series," 1880. 



