222 



EMBRYOLOGY 



posterior pair does not take place until the components of 

 each pair have completely united with each other. At the 

 point where the two prostomial discs first come together an 

 invagination is formed, the fundament of the proboscis, 

 which soon grows backward a long distance (Fig. 104 A and 



^-' '^-^ 1 So 



Fig. 104.— Diagrams to show the formation of the Nemertean (after Salensky). 



A, evaginatioiis of the oesophagus (considered by Hubkecht to be the fundaments 

 of the nephridia) ; D, intestine ; If, mouth ; N, fundament of the nervous system ; 



B, proboscis; Es, sheath of the proboscis; S,, prostomial [head-] discs; S„, meta- 

 stomial [trunk-] discs ; So, lateral organs. 



The position of the young worm in the pilidium is illus- 

 trated by Fig. 103. The larval intestine is entirely included 

 within the worm. Meanwhile the oesophagus continues to 

 pass through the body-wall of the worm, still terminating in 

 a wide opening, until at a later stage it fuses with the ecto- 

 derm of the worm and is displaced to a position relatively 

 further forward. * 



The lateral organs are said by Salensky and Hubrecht to arise in the 

 same way as the somatic discs. They originate as two invaginations of 

 the wall of the pilidium, one on either side of the oesophagus (Fig. 104 

 A, So), then grow out toward the somatic discs, and finally separate from 



