HOLOTHUROIDEA. 266 
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 very rapidly, and is 
probably defensive, the anterior portion re-forming 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 modified into 
pointed papillee. 
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. 
Ccelomic nervous tissue is developed on the perihzmal 
canals, 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 
hemoglobin. Arising from the base of the respiratory trees 
in some Holothurians there are the remarkable “Cuvierian 
