236 THE INVERTEBRATA 



break up into a tissue called mesenchyme, which is partly converted 

 into larval musculature and partly remains undifferentiated until 

 needed as raw material for the adult organs. The gut is connected 

 with the exterior by an ectodermal oesophagus, ending in a large 

 mouth on the flattened base between the lappets. Thus a creature 

 appears which has many resemblances to the trochosphere larva to be 

 described later. 



Inside this larva the young nemertean is produced (Fig. 169 A, B). 

 Five ectodermal plates (imaginal discs) sink below the surface 



Fig. 169. Pilidium larva. A, Side view of late form enclosing young nemer- 

 tean. After Korschelt and Heider. B, Frontal view of earlier stage showing 

 the imaginal discs. The anterior unpaired invagination is continued to form 

 the proboscis. After Burger, al. alimentary canal; ap.o. apical organ; 

 amn. ectoderm of the amnion; ect. ectoderm of the adult; M. mouth; mesc. 

 mesenchyme of Pilidium ; ns. nervous system ;/)r. prototroch ; rh. rhynchocoel. 



and each forms the floor of a sac. Eventually these sacs join 

 round the gut and a continuous cavity is formed separating the adult 

 inside from the larval skin (sometimes known as the amnion) which 

 is thus its protecting husk while it develops. The imaginal discs 

 join together and form the secondary or adult ectoderm. The Pilidium 

 continues to swim about with the little nemertean inside it, even when 

 the organs of the latter are developed and cilia cover its surface so 

 that the adult moves freely as if a parasite of the larva. At length it 

 bursts through the tissues of the amnion and the latter sink like a 

 discarded mantle. 



