612 A. A. W. HUBREOHT. 



tures is one of the chief factors in the composition of animal 



forms The reason for their appearance is as yet 



unknown, and the laws that control and modify them are 

 utterly obscure/^ Obscurity is not exchanged for broad day- 

 light, but something is gained when we caji see that a growth 

 of the principal organ-systems in separate and more or less 

 independent batches, which in an elongated and bilaterally 

 symmetrical animal insensibly passes into the phenomenon of 

 incipient metamery, may be of the highest value for the per- 

 sistence of the species. 



Now this is actually the way in which we find the important 

 organ-systems distributed in the lower Nemertea. And out of 

 this more irregular distribution a gradual metamery, in some 

 incipient, in others more complete, is seen to evolve within the 

 boundaries of the class. Even the nephridial system, in the 

 primitive forms provided with only one opening to the exterior, 

 participates in this tendency towards metamery, and acquires 

 a greater number of apertures, serially arranged in pairs, 

 thereby also tending towards a diminution of damage when 

 artificial division into two takes place in the nephridial region. 

 The metamery, the regular and serial repetition of parts, is 

 thus seen to be of great importance in aiding towards repair 

 after damage to a lengthened bilateral form, in the same way 

 as the radial repetition of parts facilitates repair in the Echino- 

 dermata. In both cases the destruction is only partial, the 

 other homonodynamic portions temporarily ministering, thanks 

 to their more independent relation to the injured region. 



When the faculty of repair of damage, occasioned by the 

 severing of the animal into two or more portions, has in the 

 course of generations become more and more complete, it can 

 be readily understood that it becomes at the same time a defen- 

 sive instead of being only a curative process. An animal that 

 at the approach of danger can separate in two or more parts, 

 each of them capable of reproducing an entire new animal, 

 evades this danger very effectively by doing so; whereas 

 another that is attacked in the same way and does not possess 

 this faculty, is laid hold of, shaken about, and wholly or partly 



