64 MARINE BIOLOGICAL LABORATORY. 



This theory of metamerism is an old and familiar 

 one ; it has been adopted by many eminent morpholo- 

 gists, both of the older and the newer schools. That it 

 is a plausible and fascinating hypothesis must be ad- 

 mitted. Yet there is strong reason to doubt whether 

 it can be sustained, either on general or on special 

 grounds. I have not space for a discussion of the ob- 

 jections to the theory, but I will mention a few of the 

 principal difficulties. We do not, as a matter of fact, 

 find in the lowest and most primitive annelids, as we 

 should find if the theory were true, that the somites 

 show clearly marked individuality, or any tendency to 

 become separate individuals. On the contrary, the 

 metamerism of these forms {Polygordius, etc.), is less 

 pronounced than in higher forms. It is in the highly 

 organized Polychaeta that the repetition of similar parts 

 is most marked, and in the highly modified Oligochaeta 

 that the independence of the somites is greatest. A 

 second, and much more fundamental difficulty, is that 

 the trochophore, according to Kleinenberg, has at first 

 no middle germ-layer (mesoblast). How then is it pos- 

 sible, on any theory of budding, to account for the ori- 

 gin of the trunk-mesoblast } Again, as a recent writer 

 (Meyer) has pointed out, if the somites are budded off 

 successively from the head, the anterior somites should 

 be the youngest, which is the reverse of the truth ; and 

 finally, the somites are not strictly homodynamous, since 

 the alimentary canal of the anterior and posterior 

 somites (stomodacum and proctodDsum, respectively), 

 differs entirely from that of the middle section. These 

 various objections, with others that might be given, are 

 in my opinion fatal to the entire theory. 



