the Nephridia of the Actliiotroclia. 73 



of Lonpchamps, but we dirter conccniint^ the oiii^in of the 

 lining; ot" the prcoiiil lobe and the lobe-collar nicfleiitery. 



As in the form stiulied by Loiigehainps, the anterior and 

 lateral borders of the blastopore are most active in giving 

 rise to niosodcrm-cells, and in the «!ja3trula-3ta;:je, where the 

 bhistoporo-lips have clo3e<l up somewhat, tho mesoderm-cell.^ 

 which have arisen from the anterior border of the blastopore 

 become arrani^ed into a di-tinite sac (fii^. 1). Later this sac 

 bends around the anterior end of the archontcron, so as to 

 become horseshoe-shaped, and its anterior wall becomes the 

 lining of the preoral lub^*, while its posterior wall becomes 

 the lobe-collar mesentery. At this stage the latter is complete, 

 and it remains so until the preoral lobe begins to take on the 

 shape of the hood. The larva of Phoroiiis architecta is an 

 exceptionally favourable one for studyin;^ the origin of this 

 mesentery, because of the spacious blastocoel, and there is not 

 the least doubt but that it is a delinite structure and that it 

 has not a secondary origin. 



In the larva of Phoronis architecta the lining of the collar- 

 segment does not arise as it does in the preoral lobe. It has 

 its origin largely, or possildy entirely, from isolated mesoderm- 

 cells, which come trom the lateral lips of the blastopore. 

 These cells, however, do not form a complete lining to the 

 Collar-cavity, but arrange themselves on the somatic wall, 

 leaving at least the lateral and ventral walls of the stomach 

 free from any lining. This condition continues throughout 

 the life of the Actinotrocha (fig. 2). 'I'he early stages that 

 we have been considering show no sign of the mesentery 

 which is found between the collar- and trunk-cavities of the 

 fully-formed Actinotrocha. In fact, the trunk-segment does 

 not exist at tbis time. 



I am still in doubt as to the origin of the lining of tho 

 trunk-cavity and also as to the manner in which the mesen- 

 tery arises between the latter and the collar-cavity. In larva? 

 with two pairs of tentacles the trunk-cavity is not present, 

 but in larva? with the beginnings of four pairs of tentacles it 

 makes its appearance. This stage in the development of the 

 Actinotrocha of I'horonis architrcta is very difficult to obtain, 

 liuwevcr, and I have never taken but one sisecimen (tig. 3). 



Longehamps has called attention to a figure of a young 

 Actinotrocha in llatsehek's ' Lehrbuch ' which shows tho 

 lining of the trunk-segment arising from two coelomic sacs, 

 llatschek does not describe the origin of these sacs, but 

 Longehamps asks whether or not the lining of the trunk- 

 cavity may not arise from part of the " diverticule ccto- 

 blasli(iue" (nepluidial pit). 



