ACTINOZOA. 93 
body of an ordinary sea-anemone (fig. 33, a) is a truncated 
cone or short cylinder, termed the “column,” and is of a soft 
leathery consistence. The two ends of the column are termed 
respectively the “base” and the “ disc,” the former constituting 
a kind of sucker, by means of which the animal can attach itself 
at will, whilst the mouth is placed in the centre of the latter. 
The mouth is surrounded by a flat space, destitute of ap- 
pendages, and the circumference of the disc is in turn sur- 
rounded by numerous simple tubular tentacles, arranged in 
alternating rows. The tentacles consist of both ectoderm and 
endoderm, enclosing a tube which communicates with the 
body-cavity. By the muscular contraction of the walls of the 
column, the fluid contained in the body-chambers can be forced 
into the tentacles, which can be thus protruded a great length, 
whilst they can also be usually retracted. In some cases the 
tentacles are furnished with perforations at their extremities. 
The mouth (see fig. 15, a) leads directly into the stomach, 
which is a wide membranous tube, opening by a wide aperture 
into the body-cavity below, and extending about half-way be- 
tween the mouth and the base. The wide space between the 
stomach and body-walls is subdivided into a number of separate 
compartments by radiating vertical plates, which are called the 
‘“‘mesenteries,’ and to the faces of which the reproductive 
organs are attached, in the form of reddish bands, containing | 
either ova or sperm-cells. Below the stomach, attached to the 
free edges of the mesenteries, are a series of singularly twisted 
threads or cords (fig. 15, ¢), which are filled with thread-cells, 
and are termed ‘‘craspeda.” The function of these is not well 
understood; but it is believed that in some cases they can be 
emitted through apertures which are occasionally found in 
the walls of the column. The sea-anemones are mostly to be 
found between tide-marks, in rock-pools, or on ledges of stone, 
adhering by means of the expanded base. They are not, how- 
ever, permanently fixed, but can change their place at will. In 
the nearly allied //yanthus and Arachnactis (fig. 33, 6) the base 
is tapering, and it appears that the animal spends the greater 
part of its existence in an unattached free condition. The true 
sea-anemones, as already said, are all simple, each consisting 
of a single polype; but there are closely-related forms (such 
as Zoanthus) in which the organism is compound, consist- 
ing of numerous polypes united by a creeping fleshy trunk or 
coenosarc. 
