Part II. — Stichodactylince and Zoanthece. 151 



than any other form I liave studied. This is probably a result of the excessive 

 abundance of glandular cells in both ectoderm and endoderm. The nuclei of 

 the ectodermal cells of the column-wall become aggregated towards the mesogloea, 

 and the muscular layer is distinctly recognizable where much shrinkage has not 

 taken place. No nematocysts are present. 



The mesogloea is thin, but distally is formed into irregular plaits for the support 

 of the endodermal muscle. On its ectodermal aspect it is thrown into the deep 

 ridges with wide intervening furrows, already noticed among the external 

 characters. 



The endoderm bears numerous zooxanthell?e, especially above, and also 

 scattered nematocysts. The cells are long and narrow, and the nuclei are arranged 

 in a very narrow zone. The endodermal musculature commences a little above 

 the base as a very thin, smooth layer ; distally, however, it is more developed on 

 slightly branching mesogloeal plai tings, being most concentrated just below the 

 marginal tentacles (PI. xi., fig. 3). This exaggeration should probably be regarded 

 as a diffuse endodermal spliincter muscle, continuous on the one hand with that of 

 the column-wall, and on the other with that over the smooth naked portion of the 

 disc. It is obvious that no very precise distinction can be drawn in the early 

 stages of development between a strong endodermal muscle and a concentration 

 which may be spoken of as a diffuse endodermal sphincter ; and, even in the 

 same species, there is undoubtedly much variation in the extent of the mesogloeal 

 foldings, according to the degree of retraction in which the polyp happened to be 

 preserved. 



The musculature appears to be a little better developed than was found by 

 Prof. M'^Murrich in his Bahaman specimens. In these the circular muscle of the 

 column is described as being throughout exceedingly feebly developed. The extent 

 of development here given was met with in two examples sectionized ; but in a 

 third the concentration of the muscle fibres and folding of the mesogloea was 

 observable to a much less degree, the condition evidently being about the same as 

 that found by M'Murrich. The two first specimens were somewhat infolded in 

 preservation, while the other was extended. 



Professor M'Murrich (p. 44) notes as a peculiar feature of the ectoderm of the 

 disc, marginal tentacles, and disc tentacles that nematocysts are entirely absent. 

 I have made a careful investigation of this exceptional condition, and, so far as 

 regards the results obtainable from sections, my experience agrees with that of 

 M^Murrich, but on submitting the marginal tentacles to maceration I find ecto- 

 dermal nematocysts very abundant at the tips. They are elongated, rounded at 

 each end, and the sp»iral thread is not very distinct, while the interior presents a 

 granular appearance. The endoderm also contains many nematocysts, but they 

 are of a totally difl'erent character. Maceration of the ectoderm of the disc 



Z2 



