ORDER ACTINOIDEA. oo 
number to the intervals between the fleshy lamell of the visceral 
cavity, and often equal them; and when these lamelle, in the latter 
case, increase in number, as they do with the growth of the animal, 
the new interval has soon its new tentacle, with which it communi- 
cates. 
The tentacles are commonly described as prehensile in their nature. 
In some species, they are long and flexible, and are well adapted to 
render aid in capturing the food of the polyp. They sometimes have 
the power of stinging the hand, and when without this power, they 
frequently stick closely to the fingers if handled; and when their 
prey comes within reach, they close upon it, and force it into the 
mouth, usually open for its victim. In many species, the tentacles 
are quite short, or are reduced to mere tubercles ;* and it is, therefore, 
probable that the passage of the imbibed water, contributing to the 
aeration of the fluids within, is often their more important function : 
in some instances they appear to subserve this purpose alone, being 
well adapted by their texture both to act on the external waters and 
upon the internal that may find passage through them. 
The tentacle, as above described, is the most common variety. 
There are others, both simple and branched, that are furnished with 
suctorial vesicles, like the cup-suckers of the sides, which seem to aid 
them in clinging as well as sometimes in their movements. There 
are still others, which have a minutely divided or lobed surface, and 
look as if covered with delicate embroidery, as shown on plate 5. 
This peculiar structure seems intended to enlarge the surface of these 
organs, and fit them more perfectly to aid in aeration. The circle 
of them has much resemblance to the branchial rosettes of a Holo- 
thuria. 
25. The stomach and visceral cavity occupy together the whole 
interior of the animal. The stomach is nearly cylindrical, and 
extends generally about three-fourths of the way to the base of the 
expanded animal. It has often a plaited or striated inner surface, and 
may be closed or opened by a muscular arrangement at bottom, where 
it communicates with the visceral cavity. Under the microscope, the 
surface in the A. marginata appeared smooth and covered with vibra- 
tile cilia; it was also invested with small slightly-projecting points, 
which become detached, on slight pressure, in the form of vesicles. 
* This is the case in the Actiniz, plate 2, fig. 16, and plate 4, fig. 832; and in many 
species of the genus Fungia, as shown on plates 18 and 19. 
9 
