xxvi DEVELOPMENT OF METAMERES 301 



cells ; but on that aspect of the trunk which lies on the same 

 side as the mouth i.e., to the left in Fig. 74, A and B this 

 layer has undergone a notable thickening, being now com- 

 posed of several layers of cells. This ectodermal thickening 

 is the rudiment of the ventral nerve-cord ( V. Nv. Cd\ and 

 the side of the trunk on which it appears is now definitely 

 marked out as the ventral aspect of the future worm, the 

 opposite aspect that to the right in the figures being 

 dorsal. At a later stage two ectodermal cords the cesopha- 

 geal connectives are formed, connecting the anterior end of 

 the ventral nerve-cord with the brain. Note that the two 

 divisions of the central nervous system are originally quite 

 distinct. 



The mesodermal bands, which were small and quite 

 separate in the preceding stage (Fig. 72, B and c, Msd\ 

 have now increased to such an extent as to surround com- 

 pletely the enteron and obliterate the blastocoele (Fig. 74, 

 B and B, Msd). At this stage therefore there is no body- 

 cavity in the trunk, but the space between the deric and 

 enteric epithelia is occupied by a solid mass of mesoderm. 

 In a word, the larva is at present, as far as the trunk is con- 

 cerned, triploblastic but acoelomate. 



Development continues, and the larva assumes the form 

 shown in Fig. 75, A. The trunk has undergone a great 

 increase in length and at the same time has become divided, 

 by a series of annular grooves, into segments or metameres, 

 like those of the adult worm but more distinct (compare 

 Fig. 67, D, p. 272). By following the growth of the larva 

 from the preceding to the present stage, it is seen that these 

 segments are formed from before backwards, i.e., the seg- 

 ment next the peristomium is the oldest, and new ones are 

 continually being added between the last formed and the 



