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-Ur chins 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 



