ANNULATA. 229 



is formed either by invagination or by overgrowth. In the 

 earthworm (Oligochseta) the blastopore of the gastrula forms 

 the mouth of the adult worm. In Nereis ( Poly chaeta) the 

 blastopore closes by growth, and the stomodaeum and proc- 

 todseum arise by ectodermic invaginations which finally be- 

 come continuous with the entoderm of the archenteron (Fig. 

 104, D, of Polygordius}. A ciliated, free-swimming larval 

 stage ensues, known as a trochosphere (Fig. 104, ). The 

 trochosphere may be looked upon as representing the anterior 

 or head end of the adult. The later metamorphosis to the adult 

 condition involves the reduction in size of the enormous 

 anterior region, and the growth of segments at the posterior 

 end, and is characteristic of Polychaeta. The development of 

 leeches is direct as in the Oligochseta, or in some instances it 

 might be more accurate to say that the process of metamor- 

 phosis is very much abbreviated, being completed by the time 

 of hatching. 



268. In addition to sexual reproduction many worms, par- 

 ticularly the aquatic forms, have the power of multiplying 

 by fission. In some instances this may consist of a mere 

 breaking in two, as was seen to be possible in the star-fish, 

 each part regenerating segments corresponding to those lost. 

 In other cases (Nais, Dero, etc.) zones of rapidly forming 

 segments are produced somewhere in the mid-region of the 

 body, and from this zone a new head is developed for the 

 posterior zooid and a new tail for the anterior zooid, which 

 usually become structurally complete before the separation 

 takes place (Fig. 99, z'}. 



In some of the Polychaeta (as Autolytus) a distinct alter- 

 nation of generation is found in which sexual and non-sexual 

 individuals are of very different appearance. 



When artificially mutilated the earthworm, and some other 

 types as well, may regenerate the lost portions. Groups of 

 segments of one worm may be grafted upon another, complete 

 healing taking place in such a way as to produce an apparently 

 normal worm. Pieces may be grafted on the side of another 



