CLASS HOLOTHUROIDEA 



SEA-CUCUMBERS 



THE holothurians, or sea-cucumbers, although in appearance 

 quite unlike starfishes and sea-urchins, have the character- 

 istic ambulacral zones and other features of the group. In form 

 they are cylindrical, and, when the tentacles and tube-feet are 

 retracted, resemble fat worms; when fully expanded they are 

 somewhat like sea-anemones, the tentacles forming a rosette-like 

 top. The walls of the body are tough and muscular, with small 

 calcareous deposits or spicules of various shapes in the skin. 

 The mouth is at one end, the excretory opening at the other, and 

 along the body are double rows of tube-feet. Often instead of 

 tube-feet, or together with them, are conical processes without 

 suckers. The ambulacra, when arranged in regular zones, are 

 used for locomotion only in the lines running from the madre- 

 poric plate. In some species three of the zones are near together, 

 and form a kind of sole on which the animal creeps ; again the 

 tube-feet are wholly suppressed, as in Synapta. Besides progress- 

 ing by means of these suckers, the holothurians move, as do 

 worms, by the extension and contraction of the body. The inner 

 surface of the tough membrane inclosing the body is lined with 

 powerful longitudinal and transverse muscles, by means of which 

 the creature contracts and lengthens its body and changes its 

 form in a wonderful manner. Around the mouth are tentacles, 

 which are often much branched and are used as organs of touch 

 and smell, and sometimes have an ear-sac at the base. From the 

 mouth the food-canal, making one long coil, extends to a chamber 

 (cloaca] at the other pole. The cloaca gives off a pair of much- 



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