298 POLYGORDIUS less. 



several cases which have come under our notice, be accom- 

 panied by a metamorphosis. 



The first obvious change is the elongation of the conical 

 anal region of the trochosphere into a tail-like portion 

 which may be called the trunk (Fig. 73, a). The 

 stomach (enteron), which was formerly confined to the pro- 

 and peri-stomium, has now grown for a considerable 

 distance into the trunk (b, ent\ so that the procto- 

 daeum {Pre. dm) occupies only the portion in proximity to 

 the anus. 



Important internal changes have also taken place. The 

 deric epithelium or external ectoderm is for the most part 

 composed, as in the preceding stage, of a single layer of 

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

 side as the mouth— /.^., to the left in Fig. 73, 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 oesopha- 

 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 ciuitc 

 separate in the preceding stage (Fig. 71, b and c, Msd). 

 have now increased to such an extent as completely to sur- 

 round the enteron and obliterate the blastocoele (Fig. 73, b 

 and c, 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. 



