462 T. H. MORGAN. 



the budding theory a short bilateral form is supposed to have 

 elongated by a repetition of itself to form a long chain of 

 united individuals. Other authors — Hubrecht, Lang, Meyer 

 — have supposed metaraeric repetition to have appeared in a 

 long animal which split up secondarily. Hubrecht (22, 23) 

 has supposed that a long animal, such as a Nemertian, is con- 

 tinually in danger of injury from without, and the animal has 

 met this danger by acquiring a remarkable power of regenera- 

 tion ! Those animals that had the main organs of the body 

 repeated were better able to regenerate any pieces that were 

 broken off; for such pieces would contain, in all probability, 

 all of the essential organs of the body. 



Lang^s (23) description of the origin of metameric repetition 

 from the flat-worm, Gunda segmentata, is too well known 

 to need rehearsal. 



In this elongated form the repetition of the digestive diver- 

 ticula have been the centres around which the metameres have 

 been built up. 



Meyer (29) has contended that the repetition of the meta- 

 meres is an expression of the alternate bendings of the sides of 

 the body of a free-swimming elongated worm. A pair of elon- 

 gated gonad-pouches have been broken up into a series of meta- 

 meric compartments, owing to the swimming movements of the 

 body, and around these as centres the metameres have evolved. 



The attempts to find the solution of metamerism within the 

 metameric groups have been equally unsuccessful. The 

 simpler Annelids, such as Polygordius and Protodrilus, have 

 been interpreted as archaic forms by Hatschek. This explana- 

 tion has been rejected by Kleinenberg, Eisig, and Meyer. The 

 lower Vertebrates have been equally difficult to interpret. 

 Lankester (28) found that the tail of Appendicularia showed 

 muscle-plates with corresponding ganglia in the nerve-cord. 

 The Ascidian larva has no similar structures. Brooks (8) has 

 argued that Appendicularia is a very old archaic form ; while 

 Willey (39) has interpreted the tadpole larva of the Ascidians 

 as a secondary larval form derived from a fixed ancestor. By 

 inference, therefore, Appendicularia is a sexually mature larva, 



