434 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 



figure 9 of plate 31, which is part of a cross section in the plane which, 

 in figure 4, is just below that in which the radiating tubes become 

 horizontal. The endoderm cells are here very much elongated, and 

 their transparency and the sharpness of the walls between the cells, 

 and the other chorda-like characters, are most marked at this level. 

 The change from the highly specialized endoderm of the ascending 

 region of the canal to the ordinary endoderm of the descending region 

 takes place in the horizontal region, as plate 30, figure 4, illustrates, 

 in such a way that the aboral surface of the canal is covered with 

 ordinary endoderm where its side walls still consist of the elongated 

 and specialized endoderm. This results in the formation, on the 

 top or aboral surface of each canal, of a groove, which is narrow and 

 shallow at its axial end, while it is very broad and deep peripherally, 

 as figure 4 shows. The floor of the groove consists of ordinary endo- 

 derm covered by the ectoderm of the subumbrella, and its cavity is 

 occupied by the jelly of the bell. The sides of the groove are high 

 walls of the modified endoderm covered by ordinary ectoderm. These 

 walls are nearly vertical on their inner surfaces, and rounded on their 

 outer surfaces. 



The descending regions of the radiating canals are shown at a in 

 plate 30, figures 1 and 2, and more enlarged in plate 31, figure 6. In 

 this figure, a is the cavity of the canal; b the flattened ectoderm of the 

 exumbrella; c the jelly; d the endodermal lamella, which consists of 

 flattened and rudimentary endoderm, and joins each radial canal to 

 the adjacent ones; / is the ectoderm of the subumbrella, with a well 

 marked layer of longitudinal muscles, and a thick supporting layer, c; 

 g is the endoderm of the outer or peripheral surface of the canal, con- 

 sisting of cells that are nearly cubical, and so crowded with small 

 granules that they are nearly opaque in preserved material; h is the 

 endoderm of the central or internal side of the canal. The cells are 

 columnar, and they contain numerous granules that are much larger 

 than those in the external endoderm. The endodermal lamella is 

 continuous with the internal endoderm. 



Plate 31, figure 7, is a section through the circular marginal canal, 

 a. Its wall consists of two sorts of endoderm: flattened and cubical, 

 and long and prismatic; but their relative positions are the reverse 

 of those in the radiating canals, as the cubical endoderm is on the 

 internal side and adjacent to the ectoderm of the subumbrella, and 

 the elongated endoderm adjacent to the exumbrella. This figure 



