ANNELIDA 



269 



to each ßegment of the body a pair of primitive segments be- 

 longs, these meet in the middle line of the ventral and dorsal 

 surfaces, and form there a ventral and dorsal mesentery 

 (Fig. 133 G, p. 290). In the figures 120 A and B (lateral 

 views of the larva) the primitive segments can already be 

 recognized in the form of an internal segmentation of the 

 larva. The most anterior primitive segments are the oldest, 

 the posterior ones younger. The body is seen to be already 

 considerably grown out backwards, although the head por- 

 tion has not yet diminished in circumference. Two ciliated 

 tentacles, which are 

 still very small, now 

 make their appear- 

 ance on the apical 

 plate (Fig. 120 B). 

 The originally sac- 

 like mid -gut has 

 grown in length 

 with the body, and 

 is now cylindrical in 

 shape. Very near 

 the posterior end of 

 the body, a short 

 distance in front of 

 the anus, is the pos- 

 terior ciliated band 

 (Fig. 120 B). 



While the pos- 

 terior part of the 

 body of the larva is 

 gradually passing 

 from the earlier conical into the cylindrical form, the head 

 portion attains its greatest volume, but thereafter soon 

 diminishes. The metamorphosis of the voluminous cephalic 

 vesicle into the slender cephalic process of the worm is 

 effected by the thickening and conical outgrowth of the 

 apical plate (Fig. 121 A), and by the contraction of the wall 

 of the head generally. The previously flat cells become 

 considerably thickened, whereby the circumference of the 



IR ^ 



Fig. 120.—^ and B, larvae of Polygoi-dius (after 

 Hatschbk). a, anus ; m, mouth-opening ; kn, head 

 kidney ; mes, mesodermal bands ; sp, apical plate. 



