SEA CUCUMBERS. 241 



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. 



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, and 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 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 are constantly supplied 

 with fresh water by means of the rhythmic contractions of 

 the cloaca, and seem in the adult to be respiratory in 

 function. The body fluid sometimes contains a red pig- 

 ment, said to be identical with the hemoglobin of verte- 

 brates. Arising from the base of the left respiratory tree 

 there are remarkable structures, known as Cuvierian organs, 

 consisting of numerous tubes, which are in most cases 

 glandular. The Holothurian can eject these tubes through 

 the cloaca, the wall of which is apparently ruptured in the 

 process. The tubes are very viscid, and seem to grow 

 longer in the water ; they will adhere to almost everything 

 but the Holothurian itself. 



The water vascular system may consist of a ring around 

 16 



