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



early development of the body cavities and not on the adult organization of the Actinotrocha, 

 for in the collar he finds only a dorsal mesentery (no other investigator has seen this), and in the 

 trunk only a ventral mesentery. 



While it is possible that the mesoderm arises as diverticula from the enteron, as Caldwell 

 and Masterman have described, yet Longchamps (12), who has recently reinvestigated the early 

 embryology of the form that Caldwell worked on {Phoronis kowaU vskii), denies the origin of the 

 cavity in front of the collar trunk mesentery from enteric diverticula. Ikeda (it), who recognizes 

 the "anterior diverticula" of Caldwell in the Japanese species, nevertheless holds that the body 

 cavities do not arise from anterior diverticula, but are simply produced by mesoblast cells apply- 

 ing themselves to and forming the lining of the ectoblastic and entoblastic wall. 



In the section on the mesoderm we have stated that in the embryo of Phoronis architecta we do 

 not find that the mesoderm arises from enteric diverticula, There can not be the least doubt, 

 however, that the preoral lobe at an early stage becomes lined by a sac of mesoderm cells and 

 that the wall of this sac gives rise to the mesentery. Furthermore, this sac is extended postero- 

 lateral^ into two horns winch are characteristic of the cavity of the preoral lobe, according to 

 Masterman, Ikeda, and Menon. It must be admitted, however, that this sac does not seem to 

 retain its character as a sac. but that the cells become separated and apply themselves here and 

 there to the walls of the preoral lobe. The mesentery remains intact and can not be considered 

 as a secondary structure as has been suggested by Longchamps. Although we agree with Ikeda's 

 statement that the mesentery between the lobe and the collar is incomplete laterally in the fully 

 developed Actinotrocha, yet in the Actinotrocha of Phoronis architecta, at least, it must be 

 considered as a definite mesentery. 



The fully formed Actinotrochse (Species A. and Species B.) do not show a complete epithe- 

 lial lining to the preoral lobe, but the mesoderm cells are arranged as described in the young 

 larva. 



It is stated above in tin' part on the mesoderm that we do not find that the lining of the collar 

 cavity is of enteroccelic origin in Phoronis architecta. However, in the fully formed Actino- 

 trocha there is an undoubted mesodcrmic ephithelium lining the somatic wall. This layer is very 

 conspicuous immediately before metamorphosis, because it becomes separated from the somatic 

 wall prior to becoming transformed into the ring vessel of the adult (tigs. 51;/, 51^). 



The splanchnic wall of the collar cavity in the Actinotrochse, that we have examined is devoid 

 of a mesodermal lining, and the occurrence of mesoderm cells on the wall is very infrequent. 

 This condition of affairs in the well-developed Actinotrocha is what one would expect from the 

 disposition of the mesoderm cells in the very young larvse of Phoronis architecta, where it is 

 only very seldom that any are found on the stomach wall (tig. 24). 



The absence of a mesodermal lining on the splanchnic wall of the collar cavity is made all 

 the more evident by the examination of cross sections showing the collar-trunk mesentery 

 (figs. f>lg, 51/0- When the mesentery reaches the stomach wall, instead of dividing into two 

 layers, one of which would be continued into the mesodermal lining of the stomach wall of 

 the collar cavity, it turns abruptly upon itself and becomes the lining of the stomach wall 

 of the trunk. We have never found the least indication in the collar cavity of a dorsal mesentery 

 such as Masterman (15) has described in the Actinotrocha from St. Andrews Bay. The trunk 

 cavity is lined throughout by a sac of mesodermal epithelium, and the mesentery is plainly seen 

 to be continuous with the lining of the somatic wall and with the lining of the wall of the gut. 

 The ventral mesentery of the trunk is present in Species A. and Species B., and while there 

 is no dorsal mesentery we have found indications of it in two specimens only (Species B.) at the 

 posterior end of the trunk. We can not say. however, that it has any ontogenetic significance 

 ( tigs. 18, 41/, 44;/). We have also found the ventral mesentery to be present in the Actinotrocha of 

 Phoronis sabatii ri. 



The ventral pouch fills a large part of the trunk cavity in the fully formed Actinotrocha, and 

 just before metamorphosis it frequently pushes the collar trunk mesentery well forward into the 

 collar cavity, thus making the study of the relation of the different parts quite difficult. Both 



