214 ECHINODERMATA. 



Class HoLOTHUROiDEA. Sea-Cucumbers. 



The Holothurians do not at first sight suggest the other 

 Echinoderms, for they are like plump worms, and the 

 calcareous skeleton is not prominent. But closer examination 

 shows the characteristic pentamerous symmetry, and the 

 occurrence of calcareous plates in the skin. We may think 

 of a sea-urchin as a hardened Holothurian, or of a 

 Holothurian as a soft sea-urchin with its mouth surrounded 

 by tentacles. 



They occur in almost all seas, from inconsiderable to very 

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

 organic particles from the sand. Some of them grasp little 

 things in their waving tentacles, and then plunge these 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 at leisure. 



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 " are often approxi- 

 mated on a flattened ventral sole, leaving two on the 

 convex dorsal surface, and there are other modifications of 

 form. 



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

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

 of lime scattered in the skin, 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 the nerves to the tentacles arise. 

 Sense organs are represented by the tentacles, which some- 

 times 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 into a 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 are homologous with the anal 



