350 Ralph S. Lillie 



the eiliated cells of the Oesophagus aud posteriorly with those of 

 the rectum. Posteriorly, the stomach merges g-radually in earlier 

 stages into the undifferentiated posterior portiou of the intestine; 

 later its posterior limit becomes defined, though uever with absolute 

 precision; this limit is in the tenth or eleveuth somite, where the 

 stomach passes without abrupt transitiou into the posterior narrower 

 portion of the intestine. This latter ragion (rectum) is in its anterior 

 portion corapletely lined by eiliated cells continuous with those of 

 the stomach; further backward the cilia become restricted to a 

 longitudinal groove along the ventral wall; and as the growing 

 region is neared, the cilia finally disappear, and the wall becomes 

 composed of ìarge clear cells few in number in cross section, and 

 coutaining large nuclei (Figs. 21, 24, 30, Piate 23). 



Both stomach and Oesophagus are suspended to the dorsal body- 

 wall by a mesentery in which runs the supra-intestinal blood-vessel. 

 The snb-intestinal vessel runs along the ventral surface of the inte- 

 stine, and presents a characteristic appearance from the large 

 granular chloragogen cells which invest it. Posteriorly these cells 

 merge gradually into the undifferentiated mesoderm cells of the 

 growing zone (see Figs. 9, 10, 16, 17 etc., 30, Plates 22 and 23). 



For the greater part of the above period the septa form in- 

 complete transverse partitions extendiug obliquely backward from 

 the veutro-lateral regions of the corresponding somites to the sub- 

 intestinal blood-vessel. Each septum from the third to the tenth 

 inclusive differs from the rest in being associated with the develop- 

 ment of a pair of ncphridia, Avhich arise on either side in the 

 mesoblast at the time of formation of the septum, and in continuity 

 with the posterior face of the latter. Each nephridium which 

 tlius arises soon acquires the form of a minute iutracelliilar tubule 

 in the anterior region of the somite, opeuing through the septum 

 into the body-cavity beyond by an aperture through which cilia 

 project. The two earliest formed proncphridia, those of somites IV 

 and V, persist for a short time only, and degenerate at a comjjara- 

 tively early period in the development. The remainiug six pairs 

 (in somites VI to XI inclusive) are directly transformed into the de- 

 finitive adult nepliridia. The somites behind the eleventh never 

 give rise to such jìronephridia, so far as my Observation has ex- 

 tended. It is possible that proncphridia may occasioually appear in 

 these somites, as might he expected from the fact that certain species 

 exist (e. g. A. ecaudata) in which nephridia are normally present 



