Using genetics-based methods, a multinational team of scientists said in a study published this week in the journal Nature that starfish are all head and no torso.

Starfish larvae, like those of most animals including human beings, are divided into two symmetrical sections along a line, the left side and the right. From there, however, starfish grow to have five symmetrical points instead, making it hard for scientists to compare them to those divided into two sides.

Instead of having a head, trunk and tail like bilaterally divided animals, starfish do not have a traditional torso and limbs.

“It’s as if the sea star is completely missing a trunk, and is best described as just a head crawling along the seafloor,” lead author and Stanford University scholar Laurent Formerly explained to CNN.

To figure this out, scientists studied molecular markers that indicate what part of the body the cell belongs to.

“If you strip away the skin of an animal and look at the genes involved in defining a head from a tail, the same genes code for these body regions across all groups of animals,” study co-author and Stanford scholar Christopher Lowe said in a release from the university.

What the study found is that starfish have cells corresponding to a vertebrate’s head in their middle and in the centers of their five “arms” with tail-coded cells on the perimeter. No cells in starfish showed a genetic pattern corresponding to a vertebrate’s trunk or torso.

“It seems the whole echinoderm body plan is roughly equivalent to the head in other groups of animals,” study co-author Jeff Thompson of University of Southampton said in a release from that British school.

Echinoderms are a group of animals including starfish, sea urchins and sand dollars, all of which have a five-part symmetric body layout.

“To summarize starfish anatomy, I would say it’s a mostly head-like animal with five projections, with a mouth that faces towards the ground and an anus on the opposite side that faces upwards,” Mr. Thompson told the Guardian.

• Brad Matthews can be reached at bmatthews@washingtontimes.com.