12 ARTHUR DENDY. 



similar to those of the corresponding layer in the wall of the 

 proboscis. It is, however, less distinctly muscular. In the 

 immediate neighbourhood of the tentacles it retains the 

 characters which it exhibits in the tentacles themselves, being 

 comparatively thin, and having the muscle-fibres arranged 

 radially in continuation with the longitudinal muscular layer 

 of the tentacle. Elsewhere the ectoderm is thick and very 

 densely crowded with thread-cells. 



The Endodermal Canals. — The lining epithelium of 

 the endodermal canals, directly continuous with that of the 

 gastral cavity proper, is differentiated into two very distinct 

 portions, differing greatly in histological character. The 

 canals are somewhat flattened against the wall of the float ; 

 their own outer walls form part of the thickness of the latter 

 (fig. 12), and are lined by a layer of large epithelial cells with 

 rounded club-shaped ends projecting into the lumen. These 

 cells have very large vacuoles and small round nuclei, and 

 their very darkly staining granular contents are collected 

 together in or near their swollen club-shaped ends (fig. 12, 

 E7id 0.) . They also contain darkly staining spherical globules 

 of various sizes. The epithelium forming the inner walls of 

 the endodermal canals, on the other hand, consists of a single 

 layer of smaller cells, approximately cubical in shape, with 

 small nuclei and only a small quantity of faintly staining, 

 finely granular cytoplasm (fig. 12, E)id. I.). 



The Supporting Membranes of the Float. — The thin 

 transparent sheets of membrane which subdivide the cavity of 

 the float (figs. 5 — 8, 12, Sup. Mem.) appear to have a 

 very remarkable structure and origin. Each sheet consists 

 of a thin structureless layer of mesogloea (fig. 13, Mes.), 

 thickening at the angles where the sheets meet one another. 

 Spread out on each surface of this mesoglocal sheet is a still 

 thinner layer of finely granulated, frothy-looking protoplasm, 

 containing rounded nuclei irregularly scattered through it 

 (fig. 14). No cell boundaries can be distinguished in my 

 preparations, but the protoplasm appears to form a vacuolated 

 syncytium. It may occasionally be collected or drawn 



