224 TEXTBOOK OF ZOOLOGY 



The digestive system of most sea cucumbers consists of a short 

 esophagus which is supported by a skeletal structure at the point 

 where it enters the body cavity. This structure serves as attach- 

 ment for the tentacles and retractor muscles. Following the esopha- 

 gus is a short but rather inflated stomach which leads to the long, 

 coiled intestine. This tube is partially supported by a mesentery 

 which is attached to the midventral line of the body wall. The in- 

 testine is thickened in its posterior portion to become the muscular 

 cloaca which contains the openings of the two respiratory trees. 

 In the coelom are fine longitudinal muscles that lie in the ambu- 

 lacral areas. The gonad and genital duct are in ambulacral areas. 

 They are found free at one side of the esophagus and stomach. This 

 duct opens exteriorly by a pore beside the mouth. The food of the 

 sea cucumbers is largely the organic material derived from mud 

 which is ingested. This class of animals possesses a striking power 

 of autotomy and subsequent regeneration. When they are irritated 

 or disturbed, the muscles of the body cavity contract and produce 

 internal pressure sufficient to cause either the body wall to split 

 near the anus where the viscera are ejected or the viscera are forced 

 out the mouth. Other animals, attempting to attack the sea cucum- 

 ber, are rendered helpless by becoming entajigled in the visceral 

 mass. The sea cucumbers can then regenerate the lost viscera in a 

 short time. This power to eviscerate itself is a unique character- 

 istic of the group. Representative genera of this class include ; 

 Thyone, Holothuria, Cucumaria, Leptosynapta, Aphelodactyla and 

 Caudina. 



Class Crinoidea. — Most of the sea lilies live attached by long 

 stalks, but a few are free. At the free end of the stalk are located 

 the five, many-branched arms which make up the calyx. The branches 

 of the anus are called pi^inules. In most forms some lateral pro- 

 jections, called cirri, are distributed at regular intervals along the 

 stalk. The mouth is located in the uppermost center of the calyx 

 and is surrounded by the anus. The anus is also to be found on the 

 oral side of the calyx within the enclosure made by the arms. On 

 the oral surfaces of the arms are ciliated amhidacral grooves which 

 serve to transport food to the mouth. Modified tube feet are present, 

 but they serve more as tentacles than as feet. They lack ampullae 



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