STARFISHES AND 
SEA-URCHINS. 
( Echinoiknnaia ) . 
The sub-kingdom of the Echinoderms includes Starfishes^ 
Brittle Stars, Sea-Urchitis, Sea-Cucumbers and Feather Stars. 
They are all marine and form an exceedingly well defined di- 
vision of the animal kingdom. 
The animals which we have considered so far, were biiat- 
craly symmetrical, i,e. they can be cut lengthways into two equal 
halves, a right and a left one. But in a Starfish or other Echino- 
derms the chief organs of the body are arranged like the spokes 
uf a wheel and there are as many directions in which the 
animal can be bisected, a? there are arms. Such an arrange- 
ment of the body is called radial symmetry, and we find such 
symmetry also amongst Jelly Fishes and Corals. Most Star- 
fishes have five arms, 
A further characteristic of Echinoderms (i.e. 'Spiny- 
skinned *) are the calcareous plates Which are imbedded in the 
skin and which are usually raised into projections or even 
spines. The plates may form a continuous shell, as in Sea- 
Urchins, or may be much reduced, being present in Sea- 
Cucumbers as microscopic spicules only, in the shape of 
anchors and wheels. 
A live Starfish moves about with the mouth directed 
downwards, the latter lying in the exact centre of the body. 
The vent is on the opposite, upper side. At one side of the 
vent, therefore excentrically placed, is a round sieve-like per- 
forated plate, the madreporite. Arising from the mouth are 
five grooves which run along the whole length of the five arms. 
No such grooves exist on the upper side. Each of those so- 
called ' ambulacral grooves ' contains two rows of small trans- 
parent feet, called tube-feet which are really organs both for loco- 
motion and sensation and end in small sucking disks. The madre- 
porite and the tube-feet are parts of the water-vascular system 
