NEMERTINI 221 



of Balanoglossux. Moreover, the metamorphosis of this larva is said by 

 Fewkes to be accom2)lished in a manner different from that of other 

 PiHdia. Whereas usually the larva remains intact even after the 

 maturity of th worm, and in this condition is abandoned by it, in the 

 present case the collapsed Pilidium, after the withdrawal of the Ne- 

 mertean, is said to hang to its posterior end, where it is gradually 

 resorbed, in the same way as the Pluteus larva is drawn into the body 

 of the young sea-urchin. 



After Gegenbaur had expressed the view that possibly a new animal 

 was developed within the Pilidium, this idea was more precisely defined 

 by Keohn, who maintained that regularly a young Nemertean arises from 

 the pilidium. Leuckabt und Pagenstecher were able to raise this view 

 to a certainty, for they (No. 17) followed the development of the Nemer- 

 tean inside the pilidium. The accompanying processes were then fully 

 elucidated by Metschnikoff (No. 19) and Btjtschli (No. 2). 



The formation of the !N'emerteaii in the pilidium is initi- 

 ated by the appearance of four pit-like depre.ssions of the 

 ectoderm in the region of the mouth. Externally these pre- 

 sent the appearance of round suckers, for which at one time 

 they were mistaken by JoH. Muller. As the depressions 

 become deeper they become sac-like in shape (Fig. 102 C), 

 and the wall directed toward the intestine of the larva is 

 much thicker than the outer one. The further changes of 

 the invaginations consist in their being constricted off from 

 the ectoderm, becoming considerably expanded and growing 

 around the intestine of the larva (Fig. 104 A and B). They 

 have now assumed more of a discoid shape. At the points 

 where they come together the discs fuse, and their thicker 

 wall, the one directed inwards, constitutes the superficial 

 layer of the body of the Nemertean, whereas their thin outer 

 layer forms around the body an envelope, which is known as 

 the am^iion (Fig. 103 Am). This separates from its con- 

 nection with the body of the worm, which it suri^ounds as a 

 delicate membrane. The anterior pair of discs becomes the 

 head of the Nemertean (as far back as the lateral grooves), 

 whereas the posterior pair gives rise to the ectoderm of the 

 rest of the body (Fig. 104 A and B). Consequently the an- 

 terior discs, which, moreover, are the first to fuse, are known 

 as the head- [prostomial'] discs, the posterior as the trunk- 

 [metastomiaV] discs. The union of the anterior with the 



