360 ECHINODERMATA [CH. 



The buccal tube-feet form a crown of from ten to twenty-five 

 great branched tentacles, and their different shapes are used to 

 classify the various orders of the Sea- cucumbers. Most species feed 

 on sand or mud, but one order can be described only as anglers. 

 In them the tentacles are long and delicately branched so that 

 they resemble pieces of seaweed. The animal stretches them 

 out, and they become the resting-place of numbers of the minute 

 animals which swarm in sea-water. When one tentacle has got a 

 sufficient freight it is bent round and pushed into the mouth which 

 is closed on it. It is then forcibly drawn out through the closed 

 lips so that all the living cargo is swept off. 



The organs of sex are similar in nature to those of the urchins, 

 but are represented by only one mass of tubes which all unite in a 

 common opening near the tentacle region, and it is in this region 

 that the stone-canal opens in the one or two rare cases where it 

 still opens to the exterior. Hence it appears that whereas in the 

 irregular Sea-urchins the genital openings and madreporite have 

 remained fixed while the anus has been shifted, here the anus has 

 remained in its original position while the genital opening has 

 been shifted towards the mouth. 



The Holothuroidea are divided into the following six orders : 



1. Aspidochirotae : Sea-cucumbers with shield-shaped ends to 

 the tentacles, these have also large ampullae so that they can 

 be individually retracted. With gill-trees and often Cuvierian 

 organs. 



2. Dendrochirotae : Sea-cucumbers with long delicately-branched 

 tentacles without ampullae. The whole front end of the body can 

 be pulled in by means of special muscles. Gill-trees present. 



3. Synaptidse : Sea-cucumbers in which the tentacles have two 

 rows of short branches. No tube-feet except these, the radial 

 canals having also disappeared. No gill-trees. The body-wall is 

 thin and transparent and oxygen can diffuse through it. 



4. Molpadidae : Sea-cucumbers with tentacles unbranched or 

 with two or four small lateral branches, and no other tube-feet 

 except a circle of papillae round the anus. Gill-trees present. 



5. Elasipoda : Sea-cucumbers whose tentacles have shield- 

 shaped ends drawn out into short processes. No gill-trees. The 

 tube-feet of the upper surface of the body modified into stiff 

 respiratory processes. Live only at great depths in the ocean. 



6. Pelagothuriidae : Sea-cucumbers which swim or float at 

 the surface of the sea. The buccal tentacles are the only tube-feet 



