HOLOTHUROIDEA. 255 



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

 probably defensive, the anterior portion re-forming a complete individual. 

 In some forms of Cucmnaria planet 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 modified into 

 pointed papillae. 



The body-wall is tough and muscular, consisting of 

 epidermis, dermis, and circular muscles, and there are 

 paired longitudinal muscles along each radius. 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. The 

 ring and the radial nerves are sunk below the skin. 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 

 with water by means of the rhythmic contractions of the 

 cloaca. They are respiratory, hydrostatic, and excretory. 

 The body fluid sometimes contains a red pigment like 

 haemoglobin. Arising from the base of the left respira- 

 tory tree in some Holothurians there are the remarkable 

 " Cuvierian organs," consisting of numerous tubes, 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 



