458 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



into contact over the gut near the median plane and there form a sort of mesentery 

 which later is of significance as the vertical mesentery of the right coelome. 



Histologically the only change to be noticed is the thinning of the coelome 

 wall running parallel to the enlargement. On most places there is a moderately 

 fine pavement epithelium. In the mesentery and on the ventral wall of the left 

 diverticulum (adjacent to the parietal canal) larger prismatic or cubical cells 

 are present. 



From the anterior wall of the two diverticula, as well as farther dors-ally on 

 the place of division, there arise five tube-like evaginations, which indicate the 

 beginnings of the chambered organ. At their place of origin Ihey are broadened 

 out, ampulla-like; anteriorly the lumen narrows very rapidly and soon entirely 

 disappears, so that they show merely cord-like structures, which extend very far 

 anteriorly into the region circumscribed by the second ciliated ring. All five 

 tubes surround the major axis of the embrj'^o and, passing the rudimentary colum- 

 nals at right angles to their planes, reach the terminal stem plate. When the 

 rudiments of the columnals are horseshoe-shaped they surround the chambered 

 organ and its extensions dorsally and laterally. In the fixed larvse, when the two 

 ends of the horseshoes have joined and the plates have become circular elements 

 with a large hole in the center, the chambered organ lies in the channel thus 

 formed. 



In cross section the five tubes appear arranged on the periphery of a circle; 

 mesenchyme cells appear not to occur between them. Just at their origin the 

 diameter of the circle is relatively broad, and the individual tubes are large with 

 an extensive lumen; farther anteriorly the cords into which they are drawn out 

 lie nearer together. 



Seeliger numbers the tubes from 1 to 5, 1 being the one arising ventrally 

 from the left point of the right coelome, 2 the one just dorsal to it, and 5 the one 

 at the ventral end of the right sack. 



Histologically each part of the chambered organ is a single-laj^ered tube 

 formed of pyramidal or cubical cells, the nuclei of which almost always lie in 

 the middle or in the outer ends. There appears to be no ciliation of the lumen. 

 Toward the anterior end of the animal where there is no lumen the cells become 

 longer, almost spindle-shaped. They may be diff'erentiated from the surrounding 

 mesenchyme cells by the closer crowding and by the somewhat more intensive 

 staining of the nuclei. 



Certain changes have taken place in the left coclomic sack. On both sides it 

 has extended farther anteriorly, especially on the right, and this right branch now 

 ends at the same level as the left, slightly behind the anterior end of the gut. 



Neither the anterior side of the gut and hydroccrle nor the ventral side of 

 the latter are covered by the left coelomic sack, of which the walls anteriorly 

 as well as ventrally are some distance awaj' from the walls of the others. 



Anteriorly on the ventral side the beginnings of the vestibule lie between the 

 side pieces of the parietal canal. The horseshoe-like form of this canal has become 

 more marked. The two side pieces have become quite equal and no longer lie 

 ventral to tlie right coelome throughout their entire extent. The right lies nearer 



