12 HOMOLOGIES. 
water-tubes which are their locomotive appen- . 
dages. For this reason these narrower zones are 
Portion of Sea-Urchin representing one narrow zone with a part of the broad 
zones on either side and the ab-oral area on the summit. 
called the ambulacra, while the broader zones in- 
tervening between them and supporting the 
spines are called the interambulacra. -Motion, 
however, is not the only function of these suck- 
ers; they are subservient also to respiration and 
circulation, taking in water, which is conveyed 
through them into various parts of the body. . 
The oral aperture is occupied by five sets of 
pieces, which may be called jaws, remembering 
always that here again this word signifies the 
function, and not the structure usually associated 
with the presence of jaws in the higher animals ; 
and each of these jaws terminates with a tooth, 
set in its centre. Even the mode of eating in 
these animals is controlled by their radiate struc- 
ture ; for these jaws, evenly distributed about the 
