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PHYLUM ECHINODERMA. 



Class HOLOTHUROIDEA. Sea-Cucumbers 



Cylindrical or worm-like Echinoderms, elongated in tht 

 direction of the main axis, with more or less tendency to 

 bilateral symmetry, with a usually soft or leathery skin, with 

 irregularly scattered microscopic calcareous bodies, with a 

 terminal mouth surrounded by tentacles, with a posterior anus, 

 with or without tube-feet, with no external madreporite, with 

 a muscular body wall 



The Holothurians do not at first sight suggest the other 

 Echinoderms, for they are like plump worms, and the 



FIG. 138. Spicules of Holothurians. After Semon. 



The series 1-6 shows stages in the development of an anchor and a plate 

 in a Synapta. The series A-E shows stages in the development of 

 a wheel in Chiridota, a Synaptid. 



calcareous skeleton is not prominent. But closer examina- 

 tion shows the characteristic pentamerous symmetry, and 

 the occurrence of calcareous plates in the skin. These 

 seem to be absent in the unique pelagic Pelagothuria. 



Holothurians occur in most seas, from slight to very 

 great depths. Their food consists of small animals, and of 

 organic particles from the sand. Some of them catch these 

 in their waving tentacles, which are then plunged into the 

 pharynx. The muscles of a captured Holothurian often 

 over-contract and eject the viscera at the ends or through 



