- 18 - 



Such are the general organs of locomotion of the Echinoderrns, 

 for the Sea-urchins and most Sea-cucumbers move in the same 

 manner, only, because of their different shape, the sucking-feet of 

 the Sea-urchins are spread over the whole body in five rows, 

 while in the Sea-cucumbers they are otherwise disposed. 



The sucking-feet of the Sea-urchin have another very important 

 use, which may often be seen in action in the aquarium. They 

 enable the animal to catch its prey. When an animal which 

 the Sea-urchin can eat comes near,, the latter stretches out 

 a few of its tentacles or sucking-feet until they seize the prey. 

 If the victim is not immediatly aware of the attack, it is lost, 

 for other tentacles quickly follow, and the Sea-urchin gradually 

 pushes its whole body close to its prey, which is soon bound 

 fast by hundreds of tiny fetters and brought to the vicinity of 

 its enemy's mouth. All efforts to escape are in vain, for even 

 if some of the sucking-feet are torn, others take their place. 

 So the victim soon resigns itself to its fate , and is slowly 

 dissolved and digested. 



The mouth of the Sea.-urchin, like that of the Starfish, is on 

 the underside of its body. Many Sea-urchins have very com- 

 plicated masticatory organs, which are situated in the interior 

 of its body, and enable it to grind even very hard objects. All 

 the Echinoderms in the Aquarium possess these organs. Other 

 kinds live in the sand, which they swallow, digesting the organic 

 matter therein contained and then expelling the useless re- 

 mainder. 



Starfish have no masticatory organs, but the membrane of 

 their intestine secretes such an acrid fluid that it kills the 

 animals they have swallowed, such as shell-fish and snails, fre- 

 quently also fishes and crabs. The Starfish approaches some 

 animal lying quietly at the bottom of the water, and catches 

 hold of it with its sucking-feet so suddenly that it cannot 

 escape. Starfish are really distinctive animals, and ought to be 

 destroyed in the interests of fishing. 



Most of the Sea-cucumbers (Holothuridae) nourish themsel- 

 ves by taking up sand and mud and assimilating what organic 

 matter they may contain. Others act in a different manner , 

 seldom creeping about, and generally remaining quietly attached 

 to some stone or other object. They then expand their large 

 many-branched tentacles, which are filled with water, and insert 

 them alternately into their mouths. 



The Echinodermata are divided into the four classes we have 

 mentioned: 



