12 Mr. J. E. Duerden on 
band, and rounded nuclei more sparingly in a broad zone 
below ; circular or oval-shaped vacuolar spaces are present at 
intervals. The mesoglea is thin. The endoderm bears 
zooxanthellz and forms a weak muscle. 
Column-wall.—The column-wall is extremely delicate in 
the region of the vesicles, but is thicker at the capitulum. 
The ectoderm in the latter is a regular even layer, showing 
elongated nuclei; the longitudinal muscle is developed on 
small mesogloeal foldings; nematocysts are not present, 
except near the vesicles. The mesogloea is narrow and 
slightly folded both internally and externally; it thickens 
distally and becomes finely plaited to support the weak ecto- 
dermal muscle. ‘The endoderm contains zooxanthelle ; the 
circular muscle is extremely weak. 
The vesicles are very thin-walled, except at the parts 
corresponding with the opaque bands seen externally. The 
ectoderm is here thickened and shows abundant large nemato- 
cysts narrowing at each end. The spiral thread of the 
stinging-cells is not obvious. They are arranged mostly at 
right angles to the surface, but in places are longer than the 
ectoderm is broad, and become obliquely arranged. Different 
stages in their development are seen, the earlier ones staining 
deeply. The nuclei in the thickened areas are mostly in a 
narrow peripheral zone; elsewhere the ectoderm is a very 
thin layer devoid of nematocysts; oval or rounded vacuolar 
spaces are numerous. The mesogloea is very thin. The 
endoderm is a narrow regular layer of cells containing abun- 
dant zooxanthelle. A weak ectodermal and endodermal 
musculature is present. 
Sphincter muscle (Pl. I. fig. 5).—The sphincter muscle is 
an extremely weak form of the diffuse endodermal type. It 
occurs in the capitular region as a slight concentration of the 
ordinary circular endodermal muscle-fibres. ‘The mesogloea 
is only a little plaited. 
The sphincter is scarcely more strongly developed than the 
endodermal muscle of the tentacles or of the peripheral portion 
of the disk, and, except in position, is barely distinguishable 
from them. 
Tentacles—The nuclei in the tentacles are small and, in 
sections, more restricted to the periphery, where also a zone 
of long narrow nematocysts showing the spiral thread occurs, 
and occasionally much larger examples not exhibiting any 
spiral thread. A broad band of the ectoderm next the meso- 
gloea is nearly devoid of nuclei, and does not stain with 
carmine; a well-developed longitudinal ectodermal muscle is 
present. The mesogloea is somewhat thick in places, and, with 
