THE ACTINIANS OP PORTO RICO. 
345 
The ectoderm of the tentacles is a broad layer, usually much folded in preserved specimens; the 
nematocysts are small and arranged in a peripheral zone. The longitudinal ectodermal muscle is 
strongly developed, and a thin nerve layer can be distinguished toward the proximal extremity. The 
mesogloea is narrow and thrown into long, narrow, slightly branching plaits on its ectodermal border, 
following the foldings of the ectoderm. The endoderm is loaded with zooxanthellse and presents 
very irregular internal limitations. 
The naked portion of the disk shows both the ectodermal and endodermal muscles strongly 
developed on mesogloeal processes. Stinging cells do not appear to be present in the ectoderm. 
The ectoderm of the stomodreum is strongly ciliated throughout, and displays a broad zone of 
deeply staining, elongated nuclei, among which occur abundant glandular cells and long nematocysts. 
The two gonidial grooves, though clearly distinguished in the living animal, are not strongly marked 
off from the rest of the stomodaeum histologically. They are prolonged as languettes for some distance 
below the lower boundary of the tube. The mesogloea is not appreciably thickened at the grooves, 
and is very thin throughout. 
Forty-eight pairs of mesenteries are present, two pairs of which are directives. All are perfect 
for the greater part of the length of the stomodseum, but all except the primary cycle become free 
before the termination of the stomodaeum is reached, the third cycle becoming free in advance of the 
second (fig. 32). In a small polyp only twenty-four mesenteries were present, all of which were 
complete in the upper region. Below the stomodaeum the mesenteries are arranged in well-defined 
cycles, with the formula 6 6 12 24. The retractor muscle is circumscribed, and strongly developed 
on long, narrow, branching mesogloeal plaitings. A well-developed pennon, bearing the parieto-basilar 
muscle, occurs on theother face; in the upper region it is situated near the origin of the mesentery, but 
its internal free edge becomes further removed from the periphery as the lower region is approached. 
The retractor muscle is equally developed on the twelve pairs of mesenteries constituting the first 
and second cycles, but is weaker on the members of the third and fourth cycles. 
Mesenterial filaments occur on all the mesenteries belQW the stomodteal region, both the ciliated 
and glandular streaks being well developed. 
In-the half section of a polyp, represented in fig. 29, the mesentery is viewed on its ectocoelic 
face, and the parieto-basilar muscle is indicated, extending practically the whole length of the 
polyp. The mesogloea supporting it presents a very distinct free edge. The basilar muscle is 
clearly seen, and is also represented in section in fig. 31. Both inner and outer stomata occur, the 
former large and the latter exceptionally small and situated some distance from the periphery. In 
some mesenteries the outer stoma was not represented, while in others a thinning of the mesentery was 
recognizable where the aperture should occur, but the actual perforation had not taken place. 
Appellof (1900, p. 77) has recently shown that, contrary to the generally accepted opinion, the mes- 
enterial stomata originate in the embryo by resorption of the mesenterial tissue. It was maintained 
by the Hertwigs that they represent interruptions in the growth of the mesentery which had never 
been completed. Appellof found in TJrtidna that at first the mesenteries are entire, and that it is 
only later that the perforations appear, first by resorption of the mesogloea and then of the endoderm, 
the outer stomata appearing somewhat earlier than the inner. 
In Asteraclis it would seem that the outer stomata are in process of disappearing, or rather of 
ceasing to appear. Whenever present they are exceptionally small, while the large size of the mesen- 
teries has enabled me to satisfy myself that in some cases they are altogether wanting. Where a thin, 
non-peri'orated spot occurs it is evident that the absorption of the mesenterial tissue is only partial. 
The incomplete development of the apertures may perhaps be associated with the exceptional width 
of the mesentery, in this region, dependent upon the extraordinary development of the acrorhagi. 
In one polyp sectionized male gonads were met with in great abundance, occupying a large pro- 
portion of the coelenteron. The spermaria were arranged in roughly parallel rows, but on account 
of their crowded condition the cycle or cycles of mesenteries on which they were borne could not be 
determined. « 
Asteractis expansa is apparently one of the most plentiful of the Actinians within the West Indian 
area. This is certainly the case as regards Jamaica. Its abundance in the U. S. Fish Commission’s 
collections would indicate the same for Porto Rico, while, according to McMurrich (1898, p. 233), it 
