292 
E CHINODERMS. 
tube-feet; but sometimes they end in a point, and cannot assist in locomotion, 
though they may help respiration, when they are sometimes called tentacles. 
If a single foot be touched, it immediately shrinks up, and if the touch be 
vigorous, the adjacent tube-feet probably follow its example. r lube-feet torn from 
the animal sometimes continue their waving motion, showing that this is, partly 
at least, due to muscular action. Their movements are also caused by the squeezing 
of a fluid into them; for each foot is like an indiarubber tube closed at the end, 
and passing through the test (as the shell of the sea-urchin is termed) to join with 
one main tube, which runs along under the ambulacrum in a radial direction; and 
before it joins this radial canal, each tube-foot gives off a small swelling likewise 
filled with fluid, so that when this swelling is contracted all the fluid is squeezed 
up into the foot, and pushes 
it out like the Anger of a 
glove when blown into. The 
radial canals pass along under 
the ambulacra till they join 
in a ring-canal surrounding 
the mouth. Eventually this 
circular canal is connected 
with the surrounding water 
by a canal passing right 
across the body - cavity to 
the other side of the animal, 
near the vent, where it opens 
to the exterior through a 
plate pierced with a number 
of pores. This plate is called 
the madreporite,and the canal 
/, Small swellings connected with the tube-feet; A, The radial canal with ]eadi to it-owing; to the 
which they unite ; e , Kmg-caiial into which the radial canals open ; 0 ” 
c, d, Membranous sacs that serve as reservoirs for water from radial limy deposits formed ill its 
canals ; a, Stone-canal, leading from ring-canal to the madreporite, wa pg_^pg stone-Canal This 
n; m, Mouth. 
whole system of fluid-filled 
canals is termed the water-vascular system. The foregoing description refers to 
its arrangement in a star-fish, or regular sea-urchin; but the system occurs, with 
various modifications, in all Echinoderms, and is one of the features that separate 
the group from other animals. 
The Echinoderms are also peculiar in the possession of three, or perhaps four, 
different systems of nerves, of which three, or at least two, are present at the same 
time. One system supplies the skin, the tube-feet, and the intestine ; its chief parts 
being a ring round the mouth, and radial nerves radiating therefrom. The second 
system has a similar arrangement, but lies deeper, and supplies the internal 
muscles of the body-wall. The third system, which is most fully developed in 
crinoids, starts from the other side of the body, opposite to the mouth, and supplies 
the muscles that work the arms and stem. If the arm of a star-fish be opened 
from the back, there will be seen a pair of pleated extensions from the stomach. 
If these be removed, there will be exposed a pair of orange-coloured tubes, some- 
DIAGRAM OR AMBULACRAL SYSTEM OP A STARFISH. 
