Let us take first the Sea-Urchin and examine in detail
all parts of its external structure. I shall say nothing of the
internal structure of any of these animals, because it does not affect
the comparison of their different forms and the external arrangement of
parts, which is the subject of the present article.
On the lower side is the mouth, and we may call that side and all the
parts that radiate from it the oral region. On the upper side is a
small area to which the parts converge, and which, from its position
just opposite the so-called mouth or oral opening, we may call the
_ab-oral region_. I prefer these more general terms, because, if
we speak of the mouth, we are at once reminded of the mouth in the
higher animals, and in this sense the word, as applied to the aperture
through which the Sea-Urchins receive their food, is a misnomer. Very
naturally the habit has become prevalent of naming the different parts
of animals from their function, and not from their structure; and in
all animals the aperture through which food enters the body is called
the mouth, though there is not the least structural relation between
the organs so designated, except within the limits of each different
branch or division.
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