6 G. HERBERT FOWLER. 



and vacuolated, though the layer is still apparently only one cell 

 deep. 



Some sections lower down in the polyp (Fig. 8 a), another similar 

 involution appears in the stomatodgeum, 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 mesentery (Fig. 87). 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 stomatodaeum ceases, the free edge 

 of the mesentery is enlarged into a perfectly normal filament (Fig. 89); 

 and finally (Fig. 811), the whole modification disappears suddenly, 

 the two canals meeting below ; the mesentery then presents a per- 

 fectly 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 occurs a 

 swelling due to elongation of the endoderm cells, through which 

 runs, in the mesoderm, a canal lined by ectoderm, doubled back on 

 itself, and opening at both ends into the stomatodseum, with the 

 ectoderm of which its lining is continuous. 



Of twenty-one polyps examined, seven present this modification of 

 six (and in all cases of the same six) mesenteries, namely, those 

 numbered 2, 4, 6, 7, 9, 11, according to the method employed in the 

 diagram; the other six mesenteries, 1, 3, 5, 8, 10, 12, and all the 

 twelve mesenteries of the other polyps, are perfectly normal, and 

 show no tendency to such a modification. Were it possible to 

 explain the sectional appearances by a contortion of the mesentery,, 

 the regularity with which it occurs would be sufficient proof that it 

 is a definite modification of structure, the parallel of which has yet 

 to be sought in the Anthozoa. 



The unmodified mesenteries in Type A generally die out before 

 the plane of the opening of stomatodseum into coelenteron is reached, 

 in transverse sections. If they present a filament, which is seldom 

 the case, it is of the same character as that figured (Fig. 8 11), i.e., 

 identical with that of a modified mesentery ; more frequently none 

 is present, or at most a slight endodermal swelling on the free edge. 



