110 STARFISH GALLERY. 
In a number of Echinoids and Asteroids some of the spines are 
specially modified to act as seizing-organs — the free end being- 
divided into two, three, or rarely four pieces, which are moved on 
one another by special muscles. These minute organs were regarded 
by earlier observers as parasites, and were named pedicellarice ; they 
may be movable, when they have a stalk, or the stalk may be absent 
and the valves sessile. Considerable difficulty attaches to the deter- 
mination of the use that these organs may be to their possessors ; 
but there is reason to suppose that they may act as cleansing-organs 
by removing minute particles of dirt, and as temporary organs of 
fixation, while M. Prouho has observed their use as organs of defence. 
Echinoderms move but little ; the unstalked Crinoids, if they 
cannot find stones or worm-tubes around which to attach themselves, 
swim by beating the water] with their delicate arms, five being 
raised and five depressed alternately. The Echinoid or Asteroid is 
able to move by the aid of its podia or so-called ambulacral feet, 
which become erected by being filled with water, and are then 
contracted ; by means of this contraction movement is effected ; a 
similar kind of locomotion obtains with the pedate Holothurians ; 
in the Ophiuroids the flexible arms either serve as the organs of 
movement, or act as an apparatus whereby the creature becomes 
coiled round the branches of corals (see Case 20). 
Echinoderms are often of exceedingly bright colours, as is shown 
by the pictures on the wall, and are very conspicuous objects ; this 
may, apparently, be associated with disagreeable tastes or odours ; 
sometimes they cover themselves over with seaweed, and so hide their 
brilliancy ; the spines of some forms are exceedingly painful to the 
touch, and the stout plates of some of the Goniasters must form 
admirable organs of protection. The power of restoring lost or in- 
jured parts is one of the most remarkable points in the Echinoderm 
organization (see Case 6). 
Echinoderms are of great geological age, and were very abundant 
in earlier periods of the world's history. Two groups (the Blastoids 
and Cystids) have completely disappeared, and the Stalked Crinoids 
(Lily-Encrinites) are far less common than they used to be. Echi- 
noderms are now found in all seas, and extend to great depths of 
ocean ; many of the species have exceedingly wide areas of distribu- 
tion, and most are characterized by their gregarious habits, a large 
number of specimens of a single species being generally obtained by 
the dredge. They are most abundant in the tropical seas. 
Most Echinoderms lay their eggs in the water, where the larvaa 
