Part II,—Stichodactyline and Zoanthee. 203 
In preserved specimens the ccenenchyme and column are a dull white, due to 
the included particles. 
The diameter of the retracted polyp is about 1 mm., and the height 0°5 mm. 
The species is probably the smallest Actinian known. 
ANATOMY AND HisTouoey. 
Owing to the exceptional smallness of the polyps and the inclusion of 
numerous large, silicious, sponge spicules, the anatomical study of the species is 
carried out under considerable difficulties, and characters of fundamental import- 
ance, such as the arrangement of the mesenteries and the nature of the sphincter 
muscle, remain in some uncertainty. 
The ectoderm of the base is in contact with the tissues of the sponge on the 
one hand, and on the other is scarcely distinguishable from the outer part of the 
mesogloea, numbers of its cells passing into the latter. The individual cells are 
not disposed to form a columnar epithelium, as is usually the case, but are 
rounded or irregular in shape, and both the nucleus and the cytoplasm stain 
deeply. 
The mesogloea is divisible into two portions: an outer, broader part, much 
broken up by sinuses and cell-islets; and an inner, narrow, limiting part, more 
uniform in structure, and thickening along the line of attachment of the 
mesenteries. The former broadens much in some regions, and the large 
individual cells of the cell-islets, all with deeply-staining contents, become more 
distinct from one another. The endoderm is a somewhat thick layer, and contains 
abundant zooxanthellee. ; 
The outline of the base is convex, and in vertical sections across the length of 
the colonies the proximal region of the wall of the polyp is a little expanded 
laterally, constituting a narrow ccenenchyme. ‘The mesogloal layer here becomes 
thickened, and many silicious sponge spicules are included. 
Large, perfect sponge spicules, arranged very closely in a circular manner, are 
particularly numerous in the capitular region of the column-wall (PI. x11, fig. 9), 
while calcareous sand-grains are scarce. 
The ectoderm of the column-wall is a layer of non-columnar cells, and 
medium-sized, colourless, oval nematocysts are abundant, especially in the more 
distal regions. A cuticle is also observable. The internal limitations of the ecto- 
derm are irregular, the layer passing more or less insensibly into the mesoglea. 
The latter is so crowded with cells, that it stains nearly as deeply as the ectoderm. 
Large cell-containing spaces, connected with the ectoderm, probably represent 
the encircling sinus characteristic of the genus, and met with in the two previous 
species, but, owing to the numerous inclusions, it is impossible to make out the 
