Definition of "Animal"

This past week, in my role as a substitute teacher, I covered for high school biology. This week they were studying sponges and cnidarians (jellyfish, sea nettles, hydra, and coral) -- a particularly weak area for me.

I was reminded, or learned for the first time, that sponges are the most primitive of animals. In fact, the more I read the more I felt that biologists truly had a problem figuring out where to place sponges, in the animal kingdom or the plant kingdom. They finally agreed, based on the fact that sponges do not photosynthesize, that sponges would be considered animals, since they do ingest organic material as sustenance.

A few weeks ago I outlined Richard Dawkins' The Ancestor's Tale (which you can find on this blog), in a general, superficial manner. I remembered the section on sponges and now I went back to re-read that section.

Before reading it, I wondered whether sponges might not be in a kingdom all their own (biologists won't do that; there is already a disagreement between Brits and Americans on whether there are five or six kingdoms of life and I doubt anyone wants to add an additional kingdom, made up of one "something").

But maybe I'm not too far off. Here are the critical sentences from Dawkins on sponges:

"Only animals have true Hox genes and they are always used in the same kind of way -- to specify information about position in the body, whether or not the body is neatly divided into discrete segments. Although Hox genes have not yet been found in sponges or ctenophores, all animals have them. This would encourage ... those who proposed a new definition of the very word 'animal.' Hitherto, animals were defined as opposed to plants, in a rather unsatisfactorily negative way..."

And there you have it. Maybe the definition of animals should be those living "things" that have Hox genes.

[Ctenophores (phylum) is made of a type of jellyfish (cilia), very similar to the jellyfish in the phylum Cnidarian. Some biologists lump ctenophores and cnidarians into the Cnidarian phylum.]