492 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



in many cases it has become intimately united with the aboral wall of the hydro- 

 coele ring and with the body wall. 



Since the ventral ends of the coelome pockets are still considerably separated 

 from each other, a typical mesentery is not found in the early fixed stages. But 

 when as a result of the dissolution of the horizontal mesentery the right end has 

 broken through, this increases very rapidly and comes into contact with the left 

 blind end of the oral ccelome. The wall of the latter and that of the united oral 

 and aboral ccelome lie closely together and form the vertical mesentery of the oral 

 coelome in which the stone canal runs. This mesentery runs from the margin of 

 the esophagus directed approximately toward radius I into the interradial space 

 I-V, but always reaches the body wall near radius I. 



The inner wall of the aboral ccelome enveloping the gut and the axial organ 

 can always be clearly recognized as a distinct pavement epithelium, but the outer 

 wall (the parietal layer) in various places merges so intimately with the body 

 wall that over much of its extent it is no longer demonstrable as an independent 

 layer. 



The vertical mesentery is no longer preserved as an entity. First it atrophies 

 in its central portion, and then begins to disintegrate into individual disconnected 

 cords which traverse the body cavity longitudinally and more or less diagonally. 



Meanwhile the dissolution of the horizontal mesentery has made progress. 

 In longitudinal sections it appears as a thin lamella, including nuclei in which 

 the two layers from the union of which it has arisen are no longer everywhere 

 demonstrable. The entire central part, extending between the esophagus, stomach, 

 and gut, has disintegrated into trabeculse. 



In addition to these trabeculsp, arising from the remains of the mesentery, 

 many others form independently from the coelome wall. This process first began 

 in the preceding stage, but now the trabeculse appear in much greater numbers 

 and reach a greater length. Often an entire trabecula is composed simply of a 

 single metamorphosed cell, though commonly several take part in its formation. 



With the advancement of the dissolution of the mesentery the whole ccelome 

 takes on more and more the character of a single cavity, which is lined with an 

 inner layer (the visceral layer) inclosing the internal organs and an outer layer 

 (the parietal layer) , which here and there is fused with the body wall. 



By the remnants of the vertical mesentery of the lower ccelome and the 

 lamellae of the horizontal mesentery the body cavity is now separated into entirely 

 distinct chambers, which are traversed by trabeculse in part derived from the 

 mesentery and in part of independent origin. 



CHAMBERED OKQAN. 



As in the preceding stage the chambered organ is composed of five ampullalike 

 swellings and five processes from these extending into the stem. 



The former are encompassed by a wall composed of a pavement epithelium 

 made up of a single layer of cells; but the thickness of the wall, as well as the 

 form of the swellings and the size of the lumina, is more or less variable. These 



