[ 92 1 
which, like the radial symmetry and the spiny condition of the 
skin, is a characteristic feature of the Echinoderms, From the 
madreporite descends a sliortf straight canal which joins a ring- 
like canal encircling the movJth. The 'ring canal' in its turn 
leads into five canals which run along the whole length of the 
arms and give off the closely-set tube-feet Water is taken in 
by the madreporite and driven by cilia along the canals to the 
tube-feet to keep them distended. 
Echinoderras are all marine. They are divided into five 
classes which we now proceed to consider. 
ST A R Fl S H E S (Asterotdea), 
Two parts can be distinguished in the body of a Starfish, 
the central disk and the arms which are generally five in num- 
ber. But the proportion in size between disk and arms varies 
greatly: the arjus may he so much reduced in size that the 
animal assumes the shape of a five-sided cushion. —The ambu- 
lacral grooves are always present. 
The most common Singapore Starfish is Archaster typicus. 
Its favourite habitat are sandy stretches where M may be picked 
up at low tide. Besides the normal fivf*-rayed forms specimens 
with six arms are not at all uncommon, but four-rayed ones are 
much rarer. Specimens with 4, 5 and 6 arms respectively are 
exhibited side by side. A much larger Starfish is Greasier 
fiodosus. It is brick-red in colour, and its surface is raised into 
large black projections. Cuicita mvae-gtiineae has the shape of 
a flat pentagonal cushion. The only Singapore Starfish in the 
collection which normally has more than five arms is Luidia 
maculaia. It is nine-rayed. One of the arms of the exhibited 
specimen is abnormally small, probably the result of an acci- 
dent. This species is of rare occurrence 
BRITTLE STARS (Ophiuroidea), 
The Brittle Stars have long and slender arms which are 
never absorbed into the body, like in some of the Starfishes, and 
are always distinctly marked off from the disk-like centre- The 
scientific name signifies 'Snake-tails/ whilst the popular 
name refers to the great fragility of the arms. The ambulacral 
groove has been transformed into a closed tube and is not visi- 
ble externally. The tube-feet have no sucking disks and do 
not serve any more as locomotory, but only as tactile organs. 
The arras of some species are subdivided. 
