206 J. H. ASHWORTH. 



polyp. That these cells are ectoderraic is farther shown by the fact 

 that spicules and nematocysts are found in thern in moderate numbers, 

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

 Adjacent cylinders of cells are placed in iutimate communication by 

 numerous cords of cells which traverse the mesogloea between them. 



Sections taken further down the stem shew 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 ccelenteron. As 

 they recede further and further, the rings of cells often coalesce in the 

 mesogloea at a point almost equi-distant from their ccelentera ; 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 cyliuder of cells round each ccelenteron, 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 (T5 mm. to "2 mm.) from 

 the enclosed ccelenteron. 



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

 relation to the endoderm of the ccelenteron 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 ccelenteron, probably by the later growth of the 

 mesogloea, and is interrupted at intervals for the passage of canals and 

 cords of cells which place all parts of the mesogloea in intimate com- 

 munication with the ccelentera, the canals, and the external ectoderm. 

 Bourne (1895) has shown that in Xenia timbellata these rings of cells 

 are ectodermic on account of their connection with, and general 

 resemblance to, the ectoderm of the free part of the polyp, and the 

 occurrence of spicules and nematocysts in some of them. 



The cords of cells in the mesogloea are then chiefly ectodermic, as 

 they arise from the cylinders of cells around the coelenteron, or, in the 

 outer portion of the mesogloea, migrate inwards from the inner 

 irregular surface of the external ectoderm. The cells all present a 

 similar appearance, being either rounded or rather elongated in shape, 

 with somewhat vacuolated protoplasm. The elongated cells frequently 



