208 



THE INVERTEBRATA 



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 larv^a the young nemertean is produced (Fig. 163 A, B). 

 Two ectodermal plates (imaginal discs) on each side of the mouth 

 sink below the surface and are enclosed in sacs. 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 



^mesG^ 



pr/- 



^^ect.^'---j 



\—al. 



amii." 



Fig. 163. 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 ; pr. prototroch ; rh. rhynchocoel. 



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. 



The nemerteans are classified as follows : 



Palaeonemertini. Proboscis without stylets ; cerebral ganglia and 

 lateral nerves in the ectoderm or between the two layers of muscles. 

 Carinella. 



Metanemertini. Proboscis armed with stylets; lateral nerves 

 within all the muscle layers. Tetrastemma, Geonemej'tes , Malacobdella. 



