STARFISHES 
SEA-URCHINS AND SEA-CUCUMBERS 
NTARFISHES, sea urchins, sea lilies and sea cucumbers are 
called Echinoderms. In the form of their bodies and ar- 
rangement of their organs they usually display five rays, and are 
therefore known as “radiates.” For example, most starfishes have 
five equally developed arms, 72° apart, recalling the rays of a 
conventional star. In the Echinoderms the skin usually contains a 
skeleton composed of calcareous plates of definite shapes, all hinged 
together in an orderly manner, so as to make a veritable armor 
which gives rigidity to the body, and protects the soft organs of 
the interior. In the living starfish one willsee hundreds of little 
tubular feet which arise from the grooves on the lower side of the 
arms. When the starfish is turned over upon its back these feet 
stretch out to a remarkable length and wave about, seeking to 
fasten upon something in order to right the animal. It is then 
we may see that each of these feet is a hollow tube ending in a 
cup-shaped sucker. 
Similar tube-feet will be seen in five double lines along the 
sides of the sea urchins. The mouth of the starfish is at the centre 
of the lower surface. On the upper side, and a little away from 
the centre between two arms, one will see a spongy-looking area. 
This is called the madreporic plate, and is the sieve-like entrance 
to the water-tubes of the starfish which extend down the arms and 
give rise to little bladder-like vessels one above each tube-foot. 
The contractions of these little bladders cause the tube-feet to 
elongate by pressing water out into their cavities. 
The upper surfaces of most of the starfishes are covered with 
spines, but these are much better developed in the sea urchins 
where, in addition to spines, we find calcareous pincers mounted 
upon rods, which are used to remove any injurious foreign sub- 
stance that may fall upon the body of the urchin. 
The sea cucumbers, or Holothuria, are worm-like in appearance, 
but are nevertheless closely related to starfishes and sea urchins. 
