The five small zones are perforated with regular rows of holes,
and through these perforations pass the suckers or water-tubes which
are their locomotive appendages. For this reason these narrower zones
are called the _ambulacra_, while the broader zones intervening
between them and supporting the spines are called the
_interambulacra_. Motion, however, is not the only function of
these suckers; they are subservient also to respiration and
circulation, taking in water, which is conveyed through them into
various parts of the body.
[Illustration: 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.]
The oral aperture is occupied by five plates, 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 or plates terminates in a
tooth. Even the mode of eating in these animals is controlled by their
radiate structure; for these jaws, evenly distributed about the
circular oral aperture, open to receive the prey and then are brought
together to crush it, the points meeting in the centre, thus working
concentrically, instead of moving up and down or from right to left,
as in other animals.
Pages:
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269