Part II. — Stichodactylince and Zoanthece. 203 



In preserved specimens the coenenchyme 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 Histology. 



Owing to the exceptional sroallness 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 

 mesoglcea, 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 mesoglcea 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 zooxanthellse. 



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 coenenchyme. The mesogloeal 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. xm., 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 mesoglcea. 

 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 



