ECHINODERMATA. TI5 
ORDER VII. HoLoTHuROIDEA.—In this order are comprised 
the highest of the Echinodermata, all very different in outward 
appearance from any of the forms we have hitherto considered. 
They are commonly known as sea-cucumbers or trepangs, but 
they are rare and inconspicuous animals at the best. They are 
all more or less worm-shaped or snail-like in form, and they are 
either altogether destitute of calcareous matter in the skin, or, 
with rare exceptions, have only scattered grains and spines of 
this material. Asa rule, the skin is simply leathery, and is en- 
dowed with wonderful contractility by means of powerful longi- 
tudinal and transverse muscles. In consequence of this, they 
can in many cases eject all or almost all their internal organs, 
and can sometimes divide their bodies into several parts when 
injured or alarmed. Locomotion is effected by alternate ex- 
tension or contraction of their worm-like bodies, by anchor- 
shaped spicules of lime contained in the skin, or by rows of 
Fig. 49.—Holothuroidea. Thyone papillosa (after Forbes). 
ambulacral tube-feet, like those of the sea-urchins, protruded 
through the integument. Sometimes the tube-feet are scattered 
over the whole surface of the body, and sometimes they are 
altogether absent. There is always a mouth at one extremity 
of the body, and a distinct vent at the other. The mouth is 
situated anteriorly, and is surrounded by a circlet of feathery 
tentacles (fig. 49), which are believed to be modified tube-feet. 
The water-vascular or ambulacral system is sometimes quite 
rudimentary, but in other cases it much resembles that of the 
sea-urchins, except that the madreporiform tubercle is not placed 
on the outside of the body, but hangs down freely in the 
interior of the body. In most of the Holothuroidea there are 
appended to the termination of the intestinal canal two 
much-branched tubes, which are filled with sea-water from 
