100 MEMOIRS NATIONAL ACADEMY OF SCIENCES, VOL. X, NO. 4. 



" Adult collar cavity" (Ikeda). — This cavity, which is found in the well-developed Actinotrocha, 

 undoubtedly becomes the "adult collar cavity" or supraseptal cavity of the adult, as Ikeda says. 

 In the young Phoronis it is seen as a cavity which occupies all the region anterior to the trans- 

 verse septum and which is prolonged into the tentacles. It is lined by a mesodermal epithelium 

 and coutains the ring vessel with its tentacular vessel. 



Trunk cavity <tn<l cavity of the ventral pouch. — These cavities become the infraseptal cavity 

 of the adult. 



Ventral mesentery. — The ventral mesentery of the Acti/notrocha no doubt becomes the mes 

 entery in the adult which Cori calls the " hauptmesenterium " and which Benham names the 

 "oesophageal" and "rectal" mesenteries. Mesenteries are present in the very young Phoronis 

 (just after the completion of the metamorphosis), which are found in the exact position that one 

 would expect the ventral mesentery of the Actinotrocha to assume after metamorphosis. Ikeda's 

 figures indicate that he considers the longitudinal mesenteries of the very young Phoronis to be 

 the same as the ventral mesentery of the Actinotrocha^ for he gives them the same name. 



We can not otter any observation on the origin of the lateral mesenteries of the adult except 

 that they are not present in the very young Phoronis that we have examined. They undoubtedly 

 arise later in the life history. 



Stomach diverticula. — This structure is not present in the Actinotrocha Species A., so we are 

 not able to give any information on the subject. It seems to be the general opinion among those 

 who have studied the metamorphosis that it does not persist as an organ in the adult. 



Dig, stir, areas. —While these organs persist for some little time after metamorphosis, they 

 arc not evident as organs in the adult Phoronis architecta. 



Nephridia. — Caldwell (3), Ikeda (9), Longchamps (V2). and Menon (17) have all observed the 

 change in position of the larval nephridial canals which is due to the changes taking place during 

 the critical period of the metamorphosis, and it is a quite well-established fact that the external 

 ends of the larval nephridial canals come to be situated near the anal opening. 



.lust after the critical period a cross section through the anterior end of the young Phoronis 

 cuts the transverse septum ("diaphragm," "collar trunk mesentery"), which runs obliquely and 

 passes through the supraseptal and infraseptal cavities. It shows a transverse section through the 

 nephridial canals, which are still attached to the mesentery, as in the Actinotrocha . Following 

 the sections anteriorly, the canals are seen to open into the supraseptal cavity (" larval collar 

 cavities," " ring vessel of the adult"), and they are still found in possession of their excretory 

 cells. Posteriorly the sections show that the nephridial canals leave the septum and pass between 

 the wall and the mesodermal lining of the infraseptal cavity (tig. 59). At this time their external 

 openings are situated on the lateral epidermal wall in a transverse plane which is somewhat below 

 the transverse plane of the anus, and they are by no means as near to the latter as they are in 

 the adult Phoronis. It is seen from this description that during the critical period there is very 

 little change in the structure of the larval nephridiaor in their position, although the evagination 

 of the ventral pouch and the drawing in of the ectoderm of the trunk to form the end of the. 

 rectum causes the anus to become rather closely approximated to the external nephridial openings. 



Caldwell (3) says that the whole of the larval nephridial canals remains as the paired nephridia 

 of the adult, while Ikeda thinks it probable that only the parts of the nephridial canals lying in 

 the wall of the trunk persist. He assumes that the nephridial funnels of the adult, which both 

 open into the infraseptal cavity, are secondary outgrowths of the above remnants of the nephridial 

 canals. 



As the metamorphosis continues, sections show that the excretoiy cells and that part of the 

 nephridial canals situated in the larval body cavity have become obliterated, together with the 

 portion of the nephridial canals running in the septum. While we do not wish to deny that 

 the remnants of the nephridial canals and their external openings, situated originally in the 

 trunk cavity of the Actinotrocha. become part of the nephridia of the adult, yet in the stage 

 under consideration they could not be found. So far as we know, Ikeda is the only investigator 

 who has given us figures illustrating the relation between the larval nephridia and the nephridia 

 of the adult. While his tig. 64e shows the larval nephridial canals, his tig. 66, which is a cross 



