618 INVERTEBRATE MORPHOLOGT. 



indications of Vertebrate ancestry among the Annelida. This difficulty- 

 depends upon the interpretation placed upon metamerism and the causes 

 assigned for its origin. If the ideas regarding these points advocated in 

 preceding pages (see pp. 43 and 217) of this book be accepted, the difficulty 

 seems to be practically done away with, since these ideas imply a possi- 

 bility of the independent origin of metamerism in different groups. And 

 ! indeed it has already been pointed out that such an independent origin has 

 probably occurred, the metamerism of the Annelids being probably entirely 

 unconnected with the metamerism found in the more highly-organized 

 Platyhelminths (see p. 217). 



A full discussion of the intricate problem of the origin of the Vertebrata 

 would be out of place here, but one additional point may be referred to. 

 Difficulties have always stood in the way of an homology of the Vertebrate 

 nervous system with that of the Annelids. In the latter there is a supra- 

 cesophageal cerebrum and a ventral chain, while in the former the entire 

 central system is dorsal to the digestive tract. Various theories have been 

 advanced to account for this difference, none of which have, however, 

 proved entirely satisfactory. The acceptance of an ancestry leading back 

 to Hemichordalike forms obviates this difficulty, since in these the slightly 

 differentiated nervous cord is already entirely dorsal, a future extension of 

 it and a metameric arrangement of its elements and branches in correla- 

 tion with the metamerism of the mesoderm bringing about the Vertebrate 

 condition. Furthermore, the occurrence of a central lumen in the nerve- 

 cord and the mode of its origin are essentially the same in both Verte- 

 brates and Hemichordates, a fact which in itself must be given no little 

 weight in the final determination of the question. 



III. Class Ueochoeda. 



The Urochorda, also known as the Tunicata or Ascidians, 

 are, like the other Protochordates, exclusively marine. At 

 first sight they appear to have little resemblance to such a 

 form as Amphioxus, the majority of them lacking in the adult 

 condition all trace of a notochord, though a branchial region 

 of the digestive tract is always present. In a few adult forms 

 (Appendicularians, Fig. 285), and in the larviB of all, a well-de- 

 veloped notochord is present, however, situated in a backward 

 prolongation of the body, resembling in appearance and 

 structure the tail of a young tadpole — an arrangement which 

 has suggested the name applied to the class. In the majority 

 of forms this tail disappears at the close of larval life and 

 with it the notochord also vanishes. The name Tunicate is 

 derived from the fact that the body is enclosed within an 



