REJU\'ENESCENCE IN EMBRYO AND LARVA 



421 



The larval form of the annelids, moUusks, Crustacea, and some 

 other invertebrate groups represents chiefly the head and anterior 

 regions of the body, and metamorphosis consists, not only in changes 

 in the parts already formed, but in the addition of new segments 

 from a growing region just in front of the posterior end. The fully 

 developed larva of the anneHd Nereis, for example, consists of the 

 head and the first three segments, as indicated in Fig. 197, and during 

 the transfonnation of this free-swimming form into the worm new 

 segments are added successively at the posterior end. In this and 



Fig. 197. — Trochophore larva of Nereis. After E. B. Wilson, '92 



in other related species the axial gradient, which is so clearly marked 

 during prelarval stages, becomes less and less distinct in the larva, 

 until, as metamorphosis approaches, the growing region at the 

 posterior end shows the highest metabolic rate of any part of the 

 body. These changes enable us to gain some insight into the 

 process of formation of new segments. The head-region under- 

 goes rejuvenescence and begins senescence before other parts, so 

 that in the larval stage its metabolic rate begins to decrease before 

 that of the more posterior regions. But even before its metabolic 



