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 niesoglcea is thin. The endoderm bears 

 zooxanthellas 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 zooxanthellas ; 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 tliread 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 zooxanthellte. 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 



