Studies on the Development of Larvai Nephridia. 497 



wlien the nephridial i)it is well formed. The anus forms much 

 later, breaking through from the posterior portion of the gut or 

 rectum, which has formed in turn as an outgrowth of the stomach. 

 The anus is thus a new formation, and therc is no evidence to 

 Support Caldwell's eontention that this structure is also a part of 

 the primitive streak. 



RouLE (21) has atterapted to divide the mesoblast of Piwronis 

 iuto a primary meseuchyme, and a mesoderm, correspouding roughly 

 with the larvai and definite mesoderm of Annelids. Unfortunately 

 this last structure gives rise to the "bandelettes mésodermiques" 

 which are nothing but the diverticula of the nephridial pits which 

 he has mistaken for mesoblast bands, The coelom he derives from 

 the Splitting of these bands, which as I hope to show is not the 

 case, for the early coelom is at first unpaired, which would not be 

 so if it were derived from the hollowing out of these bands. As 

 far as I have beeu able to observe no distinction cau be drawn 

 betweeu the mesoderm cells derived from the anterior and those 

 derived from the posterior portion of the larva; they are all alike, 

 and resemble in their irregulär shape and processes the larvai 

 mesenchyme of Annelids. 



3. The nephridial pit and nephridia. 



In young larvae taken from the tentacles of the adult the anal 

 or nephridial pit is well Seen as a slight flattening or depression on 

 the posterior end of the larva (pi. 31 figs. 8 and 9). From the 

 relatively transparent condition of the ectoderm in the early stages, 

 the development of the pit can be readily followed in surface views. 

 It arises as a simple inpushing of the ectoderm, in the formation 

 of which the mesoderm takes no part. In sections the pit is seen 

 (pl. 32 figs. 30, 31, 37) to project inwards and downwards 

 below the posterior portion of the gut or stomach. The thinnest 

 ])art of the pit wall is that next the stomach, and here in places it 

 is so thin as to seem almost as if opening into the blastocoel; 

 careful examination, however, shows there is never an opening at 

 this point (fig. 31). In fig. 37, which represents a coronai section through 

 a young larva of about the age represented in fig. 8, internally the 

 nephridial pit is seen to have divided into two lateral horns or 

 diverticula. From these subsequently arise the nephridia. The 

 diverticula develop as simple folds in the wall of the pit. In fig. 30 

 representing a sagittal section slightly lateral to the median line, 



