THE STRUCTURE OF XENIA HIOKSONI. 273 



hug of cells is situated at a distance of about "06 ram. — "07 

 mm. from the endoderm of the coelenteron within. These 

 cells are the ectoderm of the portion of the polyp enclosed in 

 the stem, as can be seen on examining a longitudinal section 

 through the upper part of the stem (see fig. 8), when this 

 cylinder of cells is seen to be continuous and in line with the 

 ectoderm of the free portion of the polyp. That these cells 

 are ectodermic is further shown by the fact that spicules and" 

 nematocysts are found in them in moderate numbers, especially 

 in the upper portion of the stem (figs. 8 and 9, Sp. Nem,). 

 Adjacent cylinders of cells are placed in intimate communica- 

 tion by numerous cords of cells which traverse the mesogloea 

 between them. 



Sections taken further down the stem show that the ring of 

 cells becomes less complete and less definite, and each ring 

 widens and recedes into the mesogloea a little further from its 

 coelenteron. As they recede further and further, the rings of 

 cells often coalesce in the mesogloea at a point almost equi- 

 distant from their coelentera ; and, as the longitudinal canals 

 also lie in this region, the cells come to lie in relation with, 

 and form a plexus around, the canals. There is, therefore, 

 still a cylinder of cells round each coelenteron, but the cylinder 

 is not quite so regular as in the upper portions of the stem, 

 being interrupted at frequent intervals, and is further away 

 (•15 mm. to *2 mm.) from the enclosed coelenteron. 



In the upper portion of the stem the cylinder of cells has 

 the same relation to the endoderm of the coelenteron within it, 

 as have the ectoderm and endoderm of the free portion of the 

 polyp to each other. Crossing the denser ring of mesogloea 

 between the cylinder of cells and the endoderm are cells (viz. 

 the vacuolated and granular cells, and the small nerve-cells) 

 exactly like those observed in the mesogloea of the free portion 

 of the polyp. 



Lower down the stem the cylinder of cells has been pressed 

 further away from its coelenteron, probably by the later growth 

 of the mesogloea, and is interrupted at intervals for the pas- 

 sage of canals and cords of cells which place all parts of the 



