72 



Dr. R. P. Cowles on ike Body-cavities and 



presence of a mesentery between the cavity of the preoral 

 lobe and that of the collar. Longchamps, Ikeda, and Good- 

 rich agree that this structui*e is incomplete, and the former 

 considers it to be a secondary structure. Menon, and probably 

 Masterman, on the other hand, consider it to be a complete 

 mesentery. 



I have studied the two Actinotrochce found in the harbour 

 of Beaufort, North Carolina, and I am inclined to think that 

 one of these is the Actinotrocha of Phoronis architecta. In 

 both species there is an incomplete mesentery between the 

 preoral lobe and the collar-cavities and a complete one 

 between the collar- and trunk-cavities. The trunk-cavity 

 contains a ventral mesentery, and in one species indications 

 of a dorsal mesentery, but there is no sign of a dorsal mesen- 

 tery such as Masterman describes for the collar-cavity. 



There seems to be more or less inclination on the part of 

 those who have investigated the Actinotrocha since the appear- 

 ance of Masterman^s paper to doubt the correctness of the 

 lattery's observation as to the five-fold origin of the body- 

 cavities from the enteron. 



Fig. 1. 



.TK.-p.S. 



Sagittal section through gastrula of Phoronis architecta, X 704. 

 g., ganglion ; b.l., blastopore ; m.p.s., mesodermal preoral sac. 



I do not find that the origin of the mesoderm in Phoronis 

 architecta agrees with Masterman^s account for Phoronis 

 Buskil, nor do I find that the mesoderm arises from archenteric 

 diverticula. My observations agree more closely with those 



