Structure aud Development of the Nephridia. 357 



Everything tlius points to the conclusion tbat a disintegratiou 

 and subsequeut reorganization of tlie anterior stomacli cells are in 

 progress. In earlier stag-es, the region of transformation is found 

 farther and fartlier forwards, and always at the junction of Oeso- 

 phagus and stomach. It thus moves gradually backwards as deve- 

 lopmeut proceeds, and the process of transformation apparently ceases 

 only when the definitive limits of the two regions have become 

 finaliy established. In later larvai stages, and in the adult, the 

 Oesophagus joins the stomach near the line of Separation of the sixth 

 aud seventh somites, immediately behind the oesophageal pouches, 

 These latter organs therefore do not arise until Oesophagus and 

 stomach have attained their definitive limits. 



Some space has been devoted to a description of the above 

 histolytic changes in order to make clear the manner in which the 

 final relations of the diaphragms become established. At the body- 

 wall, the line of insertion of each diaphragm corresponds to that of 

 the originai septum to which it corresponds (1''*, S""*^ and 4*''). At 

 the intestinal insertion hovrever, no such correspoudeuce can exist. 

 The whole anterior region of the alimentary canal has undergone 

 a complete alteration from its earlier conditiou; and the sections of 

 Oesophagus bounded by successive diaphragms can therefore in no 

 way correspond to the originai segmentai divisions. In Arenicola 

 the alimentary canal fails indeed to exhibit metamerism in its anterior 

 portions, becoming differentiated as a whole, without reference to 

 segmentai limits. In the body-wall, on the other band, the 

 metamerism — as indicated by the position of the setae, external 

 body-riugs, branchiae and uephropores — corresponds closely with 

 that originally laid down in early development. These facts have 

 au important hearing on the question of the possibility of rigorously 

 markiug off segmentai limits in the adult stages of metamerie 

 animals. This can be done only in the case of certain structares 

 in Arenicola: the anterior part of the intestine shows no indications 

 of metamerism. 



It will be sufficient for the purpose of this paper to give merely 

 a brief outline of the remainder of the larvai development, since its 

 details are of interest in the present paper only in so far as they 

 concern the characteristics of the nephridia which will shortly be 

 dealt with at length. As growth proceeds, the division between 

 chaetigerous and achaetous body-sections becomes well-defined, and 

 with the completion of the diaphragms and the definite establishment 



