STARFISHES 

 SEA-URCHINS AND SEA-CUCUMBERS 



^^TARFISHES, sea urchins, sea lilies and sea cucumbers are 



called Eehiuoderms. 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 slcin 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 will see 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 

 Ave 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 bj^ 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 Holotliuria, are worm-like in appearance, 

 but are nevertheless closely related to starfishes and sea urchins. 



