MONOGR.\PH OF TI^IE KXISTING CRI>'OIDS. 



471 



section has become narrowly elongate with indentations in which the horizontal 

 part of the parietal canal, the stone canal, and the esophagus lie. 



Later the horizontal position is entirely attained, and the left ccelome, which 

 by the older authors is now Imown as the oral ccplome, surrounds the esophagus, 

 the broad ring being interrupted onh' on the ventral side. It is now no longer 

 an entirely distinct ca-\nty, for it has become united with the right coelomic sack. 



During all these changes the wall remains of one layer of cells, but owing 

 to its great extension this cell layer has become a fine pavement epithelium, which, 

 however, can always be clearly differentiated from the mesenchyme. 



On the sides forming the ventral (vertical) mesentery, where the circumeso- 

 phageal ring is interrupted, the cells are, especially in younger stages, somewhat 

 higher and richer in plasma, and here an epithelium of cylindrical cells may 

 be found. 



The changes in the right ccplomic sack are much more complicated. 



This has extended itself ventrally, and anterior to the middle beyond the 

 tip of the left ccelome it has pressed forward on the right side almost as far as 

 the median plane, from which on the left side it is separated only by a branch of 

 the parietal canal. 



The foremost end of the sack from which the chambered organ arises shows 

 in section a peculiar horseshoelike figure arising from a deep fold proceeding 

 inward from the ventral side between radii IV and V which is directed dorsally 

 and a little toward the left. This place represents that on which, in the free- 

 swimming larva, there have already appeared two diverticula from the right 

 coelome, of which the median partition walls remain united to form a vertical 

 mesentery; only in the torsion of the internal organs it has become a little ad- 

 vanced ventrally. 



In this vertical septum the cells on the left side are considerabty thicker 

 than on other places, where they form a fine pavement epithelium. There is no 

 trace of a fusion of the cells with those of the mesenchyme. 



The chambered organ is essentially as pre\'iously described ; between the 

 evaginations 1 and 5 lies the deep furrow corresponding to the anterior end of 

 the vertical mesentery. As before, the most anterior ends are drawn out cordlike 

 through the circular openings in the columnals as far as the terminal stem plate. 

 In these cords cell borders are no longer recognizable; the nuclei, which stain 

 fairly well, are elongated longitudinally. 



As the wall of the vestibule, the hydrocoele and the left coelome become more 

 and more horizontal, the parietal canal on the ventral side becomes shoved more 

 and more posteriorly, so that it no longer lies on the ventral side of the gut, 

 but of the esophagus. 



Correlatively the right ccelome extends outward over the ventral side of the 

 gut, pushing anteriorly as well as posteriorly, and the vertical mesentery becomes 

 more and more elongated posteriorly. 



The two blind ends have grown out from right and left to the median 

 plane, witliout as yet having united. The right tip lies over the left, the latter 

 remaining closely appressed to the gut. a!.* ?iiii t 



142140— 21— Bull. 82 32 



