86 LIFE AND HER CHILDREN. 
of which is a trap -door /;/ (the mouth), and out of 
which open five stately arched corridors, one of which 
is shown in Fig. 37, which begin as lofty galleries 
and end in a point where a tiny window e i? set. 
The roof and floor of the corridors are built of deli- 
cate white columns and arches of lime, joined by soft 
ligaments, while the w r alls are inlaid with star-like 
plates, and within the dome, and stretching right 
out into each corridor, lies the soft body of the ani- 
mal (S, i t Fig. 37), with its digestive organs. The 
delicate telegraph of nerves, and the water-canal, 
starting from the central hall, pass like the wires 
and pipes of our houses under the floor of each cor- 
ridor, while the numberless little water-bags which 
move the regiment of feet pierce the floor, and lie in 
the corridor itself. 
And now, how does this apparatus work ? Remem- 
bering as we do that the anemone spreads out its 
tentacles by filling them with water, we shall expect 
that something of this kind also happens here, only 
that we require besides to explain how the feet cling 
so firmly to the ground, for in some cases they will 
even break off from the body sooner than release 
their hold. 
If you look carefully at the back of a starfish 
you will find a little round spot (see A, Fig. 36, and 
/;, Fig. 37) lying at one side in the angle between 
two of the rays. This spot is a little plate of lime 
pierced with fine holes just like the rose of a water- 
ing-pot, and through it sea-water carefully filtered 
passes down a tube into a hollow ring (r) round the 
animal's mouth, and this ring opens again into canals 
which pass along under each of the rays. Here then 
