480 The Tunicates, or Ascidians 



median row, ultimately at least, give rise in turn to the egg- 

 producing individuals. 



Origin of Tunicates. There can be little doubt that the 

 Tunicata form an offshoot from the primitive Chordate stock, 

 and the structure of their larva in connection with that of the 

 lancelet throws a large light on the nature of their common 

 parents. "We may conclude," says Dr. Arthur Willey, "that 

 the proximate ancestor of the Vertebrates was a free-swimming 

 animal intermediate in organization between an Ascidian tad- 

 pole and Amphioxus, possessing the dorsal mou h, hypophysis, 



FIG. 288. Adult Tunicate of the group Larvacea, Oikopleura. Family 

 Appendiculariidce. (After Fol, per Parker & Haswell.) 



and restricted notochord of the former and the myotomes, 

 ccelomic epithelium, and straight alimentary canal of the latter. 

 The ultimate or primordial ancestor of the Vertebrates would, 

 on the contrary, be a worm-like animal whose organization 

 was approximately on a level with that of the bilateral an- 

 cestors of the Echinoderms." 



Degeneration of Tunicates. There is no question, further- 

 more, Professor Ritter observes, "that most of the group has 

 undergone great degeneration in its evolutionary course. Just 

 what the starting-point was, however, is a matter on which 

 there is considerable difference of opinion among authorities. 

 According to one view, particularly championed by Professor 

 W. K. Brooks, Appendicularia is very near the ancestral form. 

 The ancestor was consequently a small, marine, free-swimming 

 creature. From this ancestor the Ascidiacea were evolved 

 largely through the influence of the attached habit of life, and 

 the tadpole stage in their development is a recapitulation of the 

 ancestral form, just as the tadpole stage in the frog's life is a 

 repetition of the fish ancestry of the frog. 



