VI 



NEMERTINEA 



123 



stomach. Finally they meet one another, fuse and coalesce, the 

 anterior and posterior on each side and the right and left on each 

 side. The imaginal discs form the skin of the future worm whilst 

 the outer walls of the coalesced invaginations form a temporary 

 envelope known as the amnion. 



Before coalescence is quite complete the organs of the future worm 

 are constructed, and as to the manner in which this is accomplished 

 we have tantalizingly little information. It appears from Salensky's 

 account (1886) that the skin of the anterior part of the animal, as far 

 back as the cephalic slits, originates from the anterior imaginal discs. 

 The posterior imaginal discs form the skin of the hinder part of the 

 body of the worm. The characteristic proboscis is formed as an 

 ectodermal invagination. The proboscis sheath originates as a solid 



aim 



oes 



Fig. 98. — Longitudinal section througli a Pilidium larva of about the age of that 

 represented in Fig. 97. (After Salensky.) 



tr, rudiment of brain ; sft, rudiment of sheatli of proboscis. 



mass of mesoderm into which the proboscis invagination projects 

 (Fig. 99). This mesoderm appears to be in close proximity to the 

 ectodermal wall of the posterior imaginal disc on each side and 

 possibly arises from it. Later the rudiment of the sheath becomes 

 hollowed out and forms a sac lined by flattened cells and filled with 

 fluid. The adult brain (br, Fig. 99) arises as a thickening of the 

 ectoderm of the anterior imaginal discs. The cephalic slits likewise 

 arise as ectodermal ingrowths, not from the imaginal discs but from 

 the larval ectoderm between anterior and posterior discs, and pouches 

 grow out from the oesophagus to meet them {pe.p, Fig. 99 A). An anus 

 must be formed, but as to how or when we have no information. 



In fact nearly all our information about this period of development 

 is based on the examination, as whole objects, of larvae fished from the 

 sea, although Salensky has to some extent applied the method of 

 sections. If once an appropriate food for the Pilidium larvae could be 



