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



We had confidently expected to find in the larva of the stage represented in fig. 24 some 

 indication of the mesentery between the collar and the trunk, but were disappointed. Although 

 the ciliated tract along which the tentacles are to arise has made its appearance and extends 

 obliquely around the body proper of the larva, indicating a line just a little above the line of 

 separation of the cavities of the trunk and collar in the older actinotrocha. yet examination 

 of sections does not show the least sign of the mesentery or its fundament. 



Ikeda (9) has found mesoderm cells connecting the nephridial canals with the splanchnic 

 walls, and he thinks they arc the first indication of the septum between the collar and trunk 

 cavities. We have not been able to find the paired masses of "hypoblast cells" which 

 Masterman (16) says give rise to the trunk cavities, although it is true that mesoderm cells 

 were found lying on the dorsal wall of the intestine. Masterman does not follow the fate of 

 these masses, but in the larva with three pairs of tentacles he speaks of "the two mesocceles 

 pushing dorsally, their walls forming a pair of conspicuous mesenteries with the walls of the 

 inetacoeles" (p. 395). 



As has been said, the masses of cells that Masterman (16) speaks of have not been found, 

 but we do not deny that the condition which he describes may exist in the larva of Phoronis 

 busMi with three pairs of tentacles. 



" Nephridial pit" (Ikeda). — A structure of the larva of Phoronis which has given rise to 

 considerable controversy is the •■nephridial pit" ("posterior, anal, ectoblastic pit," "posterior 

 diverticulum"). It seems safe to assume that such a structure exists in the young larvae of all 

 species of Phoronis. It lias been seen by Caldwell (3a) in P. kowalevskii, by Ikeda (9) in /'. ijimai, 

 by Longchamps (12) in /'. howalt vskii, by Masterman (16«) in P. buskii and /'. hippocrepia, and by 

 us in /'. architecta. Although Roule (20) has not observed the pit in /'. sabatieri, it seems 

 probable that such a structure will he found there on further investigation. It is difficult to 

 believe that Roule's understanding of the origin of the nephridia from two cell masses of 

 somatopleure symmetrically placed at the sides of the larva is correct. As stated above, we 



consider the pit to 1 f ectodermal origin (fig. 24). Further study of the structure leads us 



to agree with Ikeda that it divides into two lateral branches, each of which becomes a nephridial 

 canal of the actinotrocha (figs. 25, 26, 27, 28). In tig. 25, which is a drawing made from a living 

 larva, the canals, which in a little younger stage were practically the same diameter throughout 

 their length, have become tipped at their distal ends with a bunch of cells which, we believe, 

 are later to form the excretory cells of the nephridium. 



No positive statement concerning the origin of these cells can be made, since it is difficult 

 to obtain many larva' of /'. architecta which are old enough to show these bunches of cells in the 

 process of formation. The sections examined afford no evidence that they are formed by free 

 mesoderm cells attaching themselves to the internal ends of the nephridial canals, and we are 

 rather inclined to consider them as arising from the cells of the internal blind ends of the 

 nephridial canals (tigs. 25-28). Ikeda's description (9) of the way the nephridial tubes arise 

 from the ectodermal pit— i. e., by tin' " reevagination of the distal unpaired portion of the 

 nephridial pit"— seems to be correct. Longchamps's (12) interpretation of the change in form of 

 the "ectodermal pit" agrees quite closely with Ikeda's description. 



"Medullary plat< " (Roule).- When the young larva of Phoronis architecta has reached the 

 two-tentacle stage cross sections show that there is a definite ventral ciliated band extending from 

 the mouth to the ciliated tentacular band (tigs. 29, /<. c, <l. - ). This ventral ciliated band has 

 been observed by Roule (20) in the larva of /'. sabatieri, but it has not been described for any 

 other species. Roule has homologized it with the medullary plate or medullary groove of the 

 annelid larva. 



" Trunk cavity." — Longchamps has drawn attention to a figure of an Actinotrocha published 

 in Hatschek's" Lehrbuch der Zoologie." In this figure are represented two ctelomic sacs sur- 

 rounding the intestine. Ilatschek does not describe the origin of these sacs, but Longchamps 

 (p. 555) proposes the question, "Si les canaux ne derivaient pas des expansions laterals du 

 diverticule ectoblastique, chacun des canaux restant en rapport avec Texterieur par un orifice 



