210 HOMOLOGIES. 
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 radiating 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 ani- 
mals 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 di- 
vision. ‘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 
