STAR-FISH AND SEA-URCHINS. 259 
is provided in its membraneous walls with a number 
of annular or ring-shaped muscular fibres; when 
these fibres contract, the fluid contained in the tube 
is forced back, while, conversely, when these fibres 
relax, the fluid runs into the tube. If the contrac- 
tion of these fibres is strong, the tube shrinks up 
entirely, ve. is retracted within the body of the 
animal; but if the contraction of the fibres is not 
so strong, the tube is only shortened. If, before its 
shortening, its terminal expansion, or sucker, has 
been applied to any flat surface, the effect of the 
shortening is to cause the sucker to adhere to the 
flat surface, in consequence of the pressure of 
the surrounding sea-water being greater than that 
of the fluid within the shortened tube. In this 
way, by alternately contracting and relaxing the 
muscular fibres in the walls of a tube-foot, a Star- 
fish is able alternately to cause the terminal sucker 
to fasten upon and to leave go of any flat surface 
upon which the animal may be crawling. In other 
words, when the tube-foot is about to form its 
attachment to a flat surface, it is fully distended 
with fluid; but when the terminal sucker touches 
the flat surface, this fluid is partly withdrawn, so 
causing the sucker to adhere. 
When we dissect out one of these tube-feet, we 
find that at its base, within the body of the animal, 
it bifurcates into two branches. One of these branches 
passes immediately into a closed sac (Fig. 36, /), 
while the other passes into a large tube (Fig. 36, 4), 
which runs all the way from base to tip of the ray, 
receiving in its course similar branches from all the 
