Athro Limited, Education on the Web
 Asymmetry in Snails 
Chicoreus palmarosae
Chicoreus palmarosae

Snails are wonderfull and bizzare creatures. Chicoreus palmarosae, pictured to the left, happens to be my favorite snail. Snails do lots of fun things. There are ...... But it is not just that lots of snails have pretty and ornate shells, or that they have lots of neat behavior (like covering their world in slime) which attracts me to them, it the anatomy of their soft parts. Snails are pretty well characterised by the scientific name for their group; that is the Gastropoda, from the greek: Gastro - stomach, pod - foot. Cut open a snail and you find a solid muscular foot on which it crawls, and a soft squishy bag of viscera sitting on top of the foot. But what is really strange is that they aren't symmetrical. The right and left sides of a snail's body are different. Now on a first cut, that makes sense, as most snails live in a coiled shell. A shell that usualy isn't symmetrical either.

 Relics of Coiling 
Breathing pore on right side of Arion intermedius

External anatomy of Arion intermedius
On fungus, Hamilton NY

But then there are slugs, snails that have no shell. If having a shell made snails to be asymetrical, then slugs should be symetrical. But looking at a slug we see an oddity - they have a breathing pore on right side of head, and none on the left. Thus slugs are asymetical, just like other snails that have shells.

Nassarius with external anatomy labled

External Anatomy of the snail Thais speciosa from the Bay of Panama

Mantle cavity anatomy of Ceratostoma foliatum
Mantle cavity of Ceratostoma foliatum


You can explore the anatomy of Ceratostoma in more detail

 Relics of Torsion 

Snails are not just asymmetrical because they coil in their shells. Snails are also asymmetrical for a total different reason - Torsion.

Symetrical sea slugs

Sea slugs - snails that have lost their shell and become externally symmetrical. They swim, crawl round etc...

Saragasso weed sea slugs
Scyella - a saragasso weed sea slug. Florida.

No obvious external asymmetry. Swim with saragasso weed

Come back to slugs - reason for asymmetry is history - decended from torted shelled ancestors that had asymmetrical anatomy. Breathing pore on right side of head is a relic of history.

Part of the Athro, Limited web site.
Copyright © 1997, 1999 Athro, Limited. All Rights Reserved.
Written by Paul J. Morris and Susan F. Morris
Maintained by Athro Limited
Date Created: 30 Dec 1997
Last Updated: 26 July 1999