CLASS HOLOTHUROIDEA 
SEA-CUCUMBERS 
§ hee 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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