the Neph'idia of the Actinotrocha. 73 



of Longchamps, but we differ concerning the origin of the 

 lining of the preoral lobe and the lobe-collar mesentery. 



As in the form studied by Longchamps, the anterior and. 

 lateral borders of the blastopore are most active in giving 

 rise to mesoderm-cells, and in the gastrula-stage, where tlie 

 blastopore-lips have closed up somewhat, the mesoderm-cells 

 which have arisen from the anterior border of the blastopore 

 become arranged into a definite sac (fig. 1). Later this sac 

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

 become horseshoe-shapedj and its anterior wall becomes the 

 lining of the preoral lobe, while its posterior wall becomes 

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

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

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

 exceptionally favourable one for studying the origin of this 

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

 the least doubt but that it is a definite 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 possibly entirely, from isolated mesoderm- 

 cells, which come from 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). The 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 this time. 



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

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

 tery arises between the latter and the collar-cavity. In larvte 

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

 but in larvge with the beginnings of four pairs of tentacles it 

 makes its appearance. This stage in the development of the 

 Actinotrocha of Phoronis architecta is very difficult to obtain, 

 however, and I have never taken but one specimen (fig. 8j. 



Longchamps has called attention to a figure of a young 

 Actinotrocha in Hatschek's ' Lehrbuch ' which shows the 

 lining of the trunk-segment arising from two ccelomic sacs. 

 Hatschek does not describe the origin of these sacs, but 

 Longchamps asks whether or not the lining of the trunk- 

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

 blastique''^ (nephridial pit). 



