There are myriads of ways to ask this same question - here's another example just to illustrate the point:

How can we understand the culturing of cells outside the human body and the immortilization of some of the cancer cell lines?

The answer to this question is that wherever there is life there is spirit, or atman.

So, if the individual cell has all the characteristics of life - birth, growth, generating by products, dwindling, death - obviously the atman is present.

The basic issue for spiritualists is to understand the definition of life - spirit. Without spirit then matter will not be animated. So in the example of the worm - since both halfs go on to live and reproduce as individuals it is quite obvious that each body is animated by an atman.

Your question may be - how did the atman get there if there was only one atman prior to division and now there are two?

These are interesting theological questions and I suppose they would pose as stumbling blocks to some people whose faith is weak and tender and who need to have logical answers to everything.

Life is mysterious both to the scientist and the spiritualist. Even the best of metanarratives will not be able to adequately explain everything.

I work as a biochemist and have had plenty of time to ponder such questions. My feeling is that any cell that reproduces does so because it has life - not just the biochemical machinery to enable it to do so. That view is at odds with the general scientifiic community who understand cellular processes as chemical processes that don't require an atman to function.

What we are really talking about is consciousness. Without consciousness matter will not be animated. Just like your computer will not funtion without the required electricity.