236 ECHINODERMA. 



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 

 a side rupture ; in this way the animal may sometimes 

 escape, and the viscera can be regrown. 



In Synapta the rupture of the body takes place veiy rapidly, and is 

 probably defensive, the anterior portion reforming a complete individual. 

 In some forms of Cucumaria planci the body divides by stricture, 

 torsion, or stretching into two or three equivalent parts, each of which 

 may regenerate the whole. In this case the autotomy seems to be 

 reproductive. 



The worm-like body is often regular in form, with five 

 equidistant longitudinal bands, along which tube-feet emerge. 

 But three of these "ambulacral areas" may be approxi- 

 mated on a flattened ventral sole, leaving two on the 

 convex dorsal surface, and there are other modifications of 

 form. In many cases the tube-feet are irregularly scattered 

 over the surface. 



The walls of the body are tough and muscular, and a 

 skeleton is represented by scales, plates, wheels, and anchors 

 of lime scattered in the skin, and by plates around the gullet 

 and on a few other regions. 



The nervous system consists of a circumoral ring in 

 which the five radial nerves running in the ambulacral areas 

 unite, and from which nerves to the tentacles arise. Sense 

 organs are represented by the tentacles, which sometimes 

 have " ear-sacs " at their bases, and by tactile processes on 

 the dorsal surface of some of the creeping forms. 



From the terminal or ventral mouth, surrounded by five, 

 ten, or more tentacles, the food canal coils to the opposite 

 pole. There it expands in a cloacal chamber sometimes 

 contractile, and from this are given off in many forms a 

 pair of much branched "respiratory trees," which extend 

 forward in the body cavity. These " trees " are supplied 



