580 ‘ RADIATA. 
being extended or contracted: to a: wonderful degree. When it wishes to 
advance, it pushes forward its body, permits the bristles to hitch against the 
ground, and then, by contracting the rings together, brings itseif forward, and 
is ready for another step. As in each full-grown Earth-worm there are at 
least one hundred and twenty rings, and each ring contains eight bristles, it 
may be imagined that the hold upon the ground is very strong. 
The COMMON LEECH is almost as familiar as the earth-worm, and is one 
of a genus which furnishes the blood sucking creatures which are so largely 
COMMON LEECH.—(//tudo medicinalss.) 
used in surgery. It belongs to a large group of Annelida which have no 
projecting bristles to help them onward, and are therefore forced to proceed 
in a different manner. All these Leeches are wonderfully adapted for the 
purpose to which they are applied, their mouths being supplied with sharp 
teeth to cut the vessels, and with a sucker-like disc, so that the blood can be 
drawn from its natural channels ; while their digestive organs are little more 
than a series of sacs in which an enormous quantity of blood can be received 
and retained. 
RADIATA. 
ECHINODERMATA. 
WE now arrive at a vast and comprehensive division of living beings, 
which have no joints whatever, and no limbs, and are calied RADIATA, 
because all their parts radiate from a common centre. The structure is very 
evident in some of these beings, but in others the formation is so exceedingly 
obscure that it is only by anatomical investigation that their real position is 
discovered. 
The highest forms in this division have been gathered together in the class 
Echinodermata. This word signifies “ urchin-skinned,” and is given to the 
animals comprising it because their skins are more or less furnished with 
spines resembling those of the hedgehog. In these animals the radiate form 
is very plainly shown, some of them assuming a perfectly star-like shape, of 
which the common star-fishes of our coasts are familiar examples, In some 
