146 ANIMAL FORMS 
and a portion of the locomotor system; but some species, at 
least, renew them again. In some of the starfishes and 
brittle-stars portions of the body 
appear to be voluntarily de- 
tached and to develop into new 
individuals, and it is thought 
that such self-mutilation is a 
normal method of reproduction. 
140. Locomotor system.—One 
of the most characteristic and 
remarkable features of the echi- 
noderms is the water-vascular 
system, a series of vessels con- 
taining water which serve in the 
process of locomotion. Their 
arrangement and mode of opera- 
tion are, with slight modifica- 
tions, the same throughout the 
group, and may be readily un- 
derstood from their study in 
Fie. 92.—Sea-cucumber (Cucu- the starfish. 
maria sp.). Natural size. On the dorsal surface of a 
starfish, in the angle between 
two of the arms, is a round, slightly elevated, calcareous 
plate, the madreporie body (Fig. 95, m.p.), which under 
the microscope appears full of holes, like the “rose” of a 
watering-pot. This connects with a tube that passes to 
the opposite side of the body, where it enters a canal 
completely encircling the mouth. On this ring-canal a 
number of sac-like reservoirs with muscular walls are at- 
tached, and from it a vessel extends along the under sur- 
face of each arm from base to tip. Each of these radial 
water-mains gives off numerous lateral branches that open 
out into small reservoirs similar to those located on the 
ring-canal, and a short distance beyond communicate 
through the wall of the body with one of the numerous 
