128 STRUCTURE QE ECHINODERMATA AND ACALEPH^. 
to allow of the separation of its two halves. The mouth, 
situated at one of the poles of the shell, is surrounded by a 
very curious apparatus of jaws and teeth (fig. 69), which 
forms what is termed the “ lanternfrom the mouth com¬ 
mences the long narrow oesophagus, m, that leads to the 
stomach, w, which is merely a dilated portion of the alimen¬ 
tary tube; the continuation of this, o, q, r, forms the intestinal 
canal, which winds once round the shell, and then doubles 
back and winds in the opposite direction, terminating at the 
anal orifice, s, which is situated at the opposite pole. The 
intestine is held in its place by a double fold of” the mem¬ 
brane lining the shell, resembling the mesentery of higher 
animals; the blood is distributed over this membrane, to be 
exposed to the aerating influence of the water admitted into 
the cavity of the shell; and the water is kept in movement 
by the cilia with which the membrane is clothed. Round the 
anus, s , are seen the five branching ovaries, each of which 
discharges its contents by a distinct orifice. The circulating 
apparatus is imperfect, the blood not being impelled by a 
distinct heart; still, however, it moves in great part of its 
course through proper vessels, and not through mere chan¬ 
nels in the tissue.—In the Star-fish , however, the body is 
very much flattened; and the stomach, instead of having a 
separate intestinal tube with a distinct orifice, is a mere bag 
with a single aperture, which serves both to take in food and 
to cast forth the indigestible remains. This character will be 
found to prevail among all the inferior Radiata. 
120. The radiated structure is also well seen in the greater 
number of animals forming the group of Acalephje, or Sea* 
Nettles . Their bodies are entirely soft and jelly-like ; contain¬ 
ing so small a quantity of solid matter, that, when upon being 
taken out of the water their fluid drains away, there is 
scarcely anything left; hence they are commonly termed 
Jelly-Fish . They derive their other name of Sea-hTettles from 
the stinging power which most of them possess. They are 
formed to float freely in the water; but they do not in 
general possess any means of actively propelling themselves 
through it. The radiated arrangement is very regularly pre* 
served in some of this group, whilst it is less evident in 
others. The accompanying figure (fig. 7 0) represents one of 
the Medusa tribe, as seen floating in water. The umbrella- 
