270 



EMBRYOLOGY 



entire head is diminished, until it is not much larger than 

 the trunk. The apical plate has grown out forward in the 

 form of a cone. The ejes are more conspicuous than in 

 the larva. In the trunk the primitive segments have in- 

 creased in number, and made the 

 segmentation of the body still more 

 distinct, since thej have enlarged 

 more and have applied themselves 

 more closely to the intestinal and 

 bodj-walls. At the posterior part 

 of the trunk they are less clearly 

 expressed. These changes are 

 much more evident in the last 

 stage of development (Fig. 121 B), 

 which we introduce for comparison. 

 There the segmental constrictions 

 of the intestine cause the metame- 

 rism to be still more distinct. The 

 cephalic vesicle and the vibratile 

 apparatus have already entirely 

 disappeared in this stage ; and we 

 have now before us in its chief 

 features the adult worm, although 

 it has not yet reached its complete 

 development. The worm gives up 

 the larval mode of life, that of 

 floating upright in the Avater, and 

 adapts itself to locomotion by 

 creeping. The papillae, which 

 make their appearance in front of 

 the posterior band of cilia, which 

 has now disappeared (Fig. 121), 

 serve the worm for the purpose of 

 attachment. 



The Different Larval Forms. 

 — PoJygordius was selected as an 

 example because it shows in a par- 

 ticularly instructive manner the transition of the larva into 

 the worm. It does not show, however, the ordinary condition 



Fig. 121.—^ and B, larvse of 

 Polygordius (after Hatschkk). 

 a, anal opening; m, mouth- 

 opening ; fcn, head kidney. 



