6 G. HERBERT FOWLER. 



mesenteries^ and a stomatodseura which is a simple invagina- 

 tion of the external body wall. A little way down in the 

 polyp, six of the mesenteries, in every case the same six, 

 assume a curious modification of structure, which will be 

 described first as seen in a series of transverse sections. Fig. 8 

 represents the characteristic features of a polyp of this type ; 

 the mesenteries numbered 2, 4, 6, 7,9, 11 are those which 

 undergo modification, and are diagrams of a series of drawings 

 made from the same mesentery with camera lucida at different 

 heights. 



There appears first (fig, 8.2) an involution of the stomato- 

 dseum directed towards the mesentery, on the floor of which 

 the ectodermic cells are long, but shorter at the sides. By 

 fusion of the mesoderm and obliteration of the ectoderm on 

 each side of this involution, a small canal with a definite 

 lumen is found to be pinched off, and to lie enclosed in the 

 mesoderm lamella of the mesentery (fig. 8.4). In the neigh- 

 bourhood of this involution, the endodermic cells lining the 

 mesenterial chamber become enormously lengthened and vacuo- 

 lated, though the layer is still apparently only one cell deep. 



Some sections lower down in the polyp (fig. 8 ), another 

 similar involution appears in the stomatodseum, in which the 

 ectodermic cells are short on the floor, but pass into deeper 

 ones at the sides ; this similarly results in the enclosure of 

 what appears to be a second canal in the centre of the mesen- 

 tery (fig. 8.7). In the first canal, as is shown in the diagram, 

 the longer ectoderm cells face towards the stomatodseum ; in 

 the second away from it. 



Further down yet, where the stomatodseum ceases, the free 

 edge of the mesentery is enlarged into a perfectly normal 

 filament (fig. 8.9); and finally (fig. 8.11), the whole modifica- 

 tion disappears suddenly, the two canals meeting below ; the 

 mesentery then presents a perfectly normal appearance, namely, 

 a mesoderm lamella with a layer of small endodermic cubical 

 cells on each side of it, and bearing the usual filament. 



The compilation of these sections, which I have attempted 

 to express in fig. 6 m, shows that on an ordinary mesentery 



