344 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
another at the outer margin, where they 'overhang. They bear tubercles and irregular short outgrowths, 
some of which become minutely dendriform, and are provided with rounded tubercles and short 
finger-shaped processes. The outgrowths are a little closer and more numerous at and toward the 
sides and ends of the acrorhagi, and are sometimes almost absent along the middle. The complete 
zone consists of three cycles: (a) A primary series, twelve in number, extending practically as far as 
the base of the outermost cycle of tentacles, and occupying the same radii as the first and second 
cycles of tentacles; centrally, as well as at the margin, they may be produced so as to be partly free 
from the rest of the disk. ( b ) A secondary series, also twelve in number, extending centripetally a 
little more than half the length of the first, and corresponding with the third cycle of tentacles, 
(c) A tertiary series of twenty-four, opposite the fourth cycle of tentacles, generally narrower and 
shorter than the last, with the tubercular and frondose outgrowths not as well developed. In the living 
condition the individual outgrowths at the centripetal end of the second cycle occasionally become 
much enlarged, inflated, and lighter in color, adding a peculiar character to the disk. The number 
may vary at times, an arrangement in fourteen instead of twelve being counted in one polyp. In 
preserved polyps the disposition of the frondose areas in cycles is by no means so readily recognized 
as in the living condition. 
Comparing the acrorhagi of Asteractis with those of a typical Bunodid, such as B. granulifera, it 
is found that the papilla;, tubercles, etc., occur on the upper aspect of the organs in the former, 
while in the latter they are restricted to the lower aspect, the upper surface being quite smooth. The 
acrorhagi of most Bunodids bear stinging cells, while these are absent from Asteractis. 
The tentacles are short, entacmseous, acuminate, broad at the base and thence narrow rapidly, 
smooth, generally forty-eight in number, and arranged in four cycles according to the formula 
6 6 12 24. The disk and tentacles may be completely infolded. The naked portion of the disk is 
flat, smooth, and occupies but a small proportion of the total apparent disk. The mouth is oval and 
the gonidial grooves are strongly marked. The stomodseum is capable of complete eversion, and under 
uncongenial circumstances the animal may even turn itself inside out. 
The coloration varies much in different parts of the same polyp, and also in individual polyps. 
The base is pellucid and white. The column is generally cream-colored below, becoming a lighter or 
darker olive above; sometimes the lower part displays irregular thin patches of crimson or orange; 
the verructe usually exhibit white centers. The colors on the disk vary somewhat according to the 
nature of the ground. The most usual condition is a pale olive, with the frond tubercles opaque 
white or grayish. The fronds may also be a dark olive, green, brown, or blackish brown. The 
tentacles are usually lighter or darker shades of olive or brown, with a green or crimson iridescence; 
seveial oval patches of opaque white, each with a clear center, occur on the inner surface. The disk 
may be flecked in various ways with opaque white or cream, often with irregular patches of crimson 
or green, especially around the mouth. 
The height of the column varies from 4 to 6 cm.; it may even reach as much as 8.2 cm. The 
diameter is likewise inconstant; on full expansion it is about 3 cm. in the middle. The length of the 
inner tentacles is 1.5 cm. The diameter of the whole disk is usually 6 or 7 cm. 
Anatomy and Histology . — The column-wall is of only moderate thickness. The ectoderm is much 
folded in contracted specimens, and no ectodermal musculature is recognizable. The mesoglcea is 
narrow, and exhibits a fibrous appearance; abundant connective-tissue cells are scattered throughout. 
The endoderm is very deep, and where the mesenteries are closely arranged becomes triangular in 
transverse sections. Zooxanthellse are abundant. The endodermal muscle is well developed on fine 
branching mesoghcal plaitings, which in retracted polyps are arranged somewhat in groups. A section 
through a verruca reveals that the mesoglcea is elevated on each side (fig. 33). 
The acrorhagi with their dendriform and tubercular outgrowths are extremely thin-walled 
structures, without any peculiar histological modifications. The mesoglcea is scarcely distinguishable 
as a distinct layer, and the ectoderm is much thinner than elsewhere and devoid of nematoblasts. A 
slight endodermal muscle occurs. 
The sphincter (pi. vm, fig. 30) is a small but typical circumscribed endodermal muscle, such 
as is characteristic of the Bunodactidx. As in that family, it occurs between the acrorhagi and the 
origin of the outer row of tentacles. It is oval in shape and subpedunculated, and the mesogloeal 
plaitings are narrow and much branched. 
