3°8 
E CHINODERMS. 
they readily avail themselves. Most walk rather than creep, raising themselves 
on their five arms as upon legs; stretching out one or two arms in front, and 
drawing the rest of their body in the same direction. Even in a state of repose, 
the arms alone touch the ground, the disc remaining raised above it In other 
forms, however, the rays of the body undulate laterally, and produce a creeping 
serpentine movement. Rondelet wrote that the common brittle-star creeps by the 
flexuous movement of its rays in the manner of serpents, and, placed on dry land, 
never ceases to move them, until it casts them off in pieces, which, although 
separate, move by bendings, like parts of worms and the cut-off tails of lizards. 
The little Amphiurci, which lives under stones, among the roots of seaweed, 
can turn its arms very quickly around its disc, and so form itself into a little 
ball; thus, if it be disturbed, it can roll and sink quickly into deeper parts of the 
water. Sometimes ophiurids are seen to progress by a kind of rowing motion of 
the arms. 
The Sea-Urchins, —Class Echinoidea. 
The sea-urchins are the best known, as they are the most numerous of all 
echinoderms. The annexed illustration shows the test or shell of the egg-urchin, 
with the spines on the right side, but scraped away from the left. The plates of 
the test are seen to be covered with rounded tubercles of various sizes, and it is to 
these that the spines are attached by 
a ball-and-socket joint, surrounded by 
muscles that can move the spines in 
any direction. The tubercles do not, 
however, cover the whole test indis¬ 
criminately, but are disposed chiefly in 
five broad zones, extending from one 
pole to the other. Alternating with 
these are five narrower zones, bearing 
smaller and fewer tubercles, and pierced 
by small holes arranged in regular rows. 
Through these holes pass the tube-feet, 
which are all provided with suckers 
at the end. These latter zones are, 
therefore, the ambulacral zones; one 
of them being seen in the middle of the illustration. The other zones are called 
interambulacral, and one of them is shown on the left of the same illustration. All 
the zones converge towards the summit of the test, where the vent is situated 
in a circular space covered with membrane. This membrane contains a few 
irregular granules, and is surrounded by five large interradially placed plates, 
pierced by the ducts of the generative glands. One is also pierced by a large 
number of small water-pores, and is called the madreporite. Outside these five 
plates, and alternating with them, are five other plates, each situated at the top of 
an ambulacral zone, and pierced by the unpaired tentacles, which terminate the 
water-canals, and represent the unpaired tentacles near the eye at the ends of the 
arms of a star-fish. At the other pole of the body is another membrane, sur- 
TEST OF EDIBLE SEA-URCHIN, WITH THE SPINES REMOVED 
FROM THE LEFT-HAND HALF (liat. size). 
