372 



THE ANIMAL KINGDOM 



vegetation using the rays as prehensile organs and they can swim. In 

 swimming the ten arms are raised and lowered as fast as 100 times a 

 minute. The two primary branches of each of the five rays alternate, so 

 that while arms 1, 3, 5, 7 and 9 are moving up, arms 2, 4, 6, 8 and 10 are 

 moving down. The many tiny branches are folded against the arm as it 

 is raised, and are spread out during the down swing. A feather star may 

 swim 15 feet in one minute. 



169. Class Holothuroidea, the Sea Cucumbers 



Sea cucumbers (Fig. 19.5 D) creep or burrow in the sand and mud. 

 The calcareous plates beneath their epidermis are microscopically small 

 and the body wall is soft and flexible. The body is elongated between 

 mouth and anus and usually one side becomes the permanent lower 

 side so that the radial symmetry is imperfect. Five rows of tube feet 

 extend from mouth to anus indicating the five ambulacral areas. Often 

 only the tube feet of the three lower rows have suckers and are used for 

 creeping. The tube feet surrounding the mouth are modified to form a 

 circle of branched tentacles. The tentacles in most species are covered 

 with mucus and extended into the water as a trap for small organisms. 

 Periodically each tentacle is bent into the mouth and wiped clean. A few 

 species use the tentacles for shovelling mud and debris into the mouth. 



In many sea cucumbers (Fig. 19.11) the rectum has a pair of large, 

 much branched diverticula that extend into the body cavity. These are 

 the respiratory trees. Rhythmically the anus opens and water is drawn 

 into the rectum. Then the anus closes and the rectum contracts, forcing 

 water into the trees. This may be repeated several times, filling the trees 

 more and more. Finally the anus opens and the whole body contracts, 

 expelling all the water. 



Intestine 



•Ri-n^ vesicle 

 ■Rin^ canal 



Stomach 



Mesentery 



Anus — i 

 Reclui 



An-iiulacrum 



Respi ralory 

 trz.e- 



Madreporite. 



Tizntacle 

 •Aristotks 

 lantern. 



•Lante.-m 

 muscle 



^^iS;5t^?j5JS07D7' 



Figure 19.11. The sea cucumber, Thy one briareus, cut open along one side. The 

 digestive tract has been moved to one side to show the respiratory trees, retractor muscles 

 of the anterior end, and the internal surface of the body wall with its five ambulacra. In 

 holothurians the madreporite lies in the body cavity, so that the water vascular system 

 is not filled with sea water, but with coelomic fluid. 



