To speak of these opposite regions in the
Sea-Urchin as the upper and lower sides would equally mislead us,
since, as we have seen, there is, properly speaking, no above and
below, no right and left sides, no front and hind extremities in these
animals, all parts being evenly distributed around a vertical axis. I
will, therefore, although it has been my wish to avoid technicalities
as much as possible in these papers, make use of the unfamiliar terms
oral and ab-oral regions, to indicate the mouth with the parts
diverging from it and the opposite area towards which all these parts
converge. [Footnote: When reference is made to the whole structure,
including the internal organs as well as the solid parts of the
surface, the terms _actinal_ and _ab-actinal_ are preferable
to oral and ab-oral.]
[Illustration: Sea-Urchin seen from the oral side, showing the zones
with the spines and suckers; for the ab-oral side, on the summit of
which the zones unite, see February Number, p. 216.]
The whole surface of the animal is divided by zones,--ten in number,
five broader ones alternating with five narrower ones. The five broad
zones are composed of large plates on which are the most prominent
spines, attached to tubercles that remain on the surface even when the
spines drop off after death, and mark the places where the spines have
been.
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