# Which animals do not have external bilateral symmetry? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [October 5, 2020, 12:34pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259 "2020-10-05T12:34:15Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Jackknifed\_Juggernaut](https://avatars.discourse-cdn.com/v4/letter/j/9de053/32.png) [@Jackknifed\_Juggernaut](https://boards.straightdope.com/u/Jackknifed_Juggernaut)\ **Post date:** [October 5, 2020, 12:34pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/1 "2020-10-05T12:34:15Z")
In the column about why internal organs are not symmetric within the body, Cecil mentions that 99% of animal species are externally symmetric. Which ones are not? > **[Why aren’t internal organs symmetrical?](https://www.straightdope.com/21344489/why-aren-t-internal-organs-symmetrical)** ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [October 5, 2020, 1:58pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/2 "2020-10-05T13:58:32Z")
My first thought would be animals with spiral shells. ---
**Author:** ![tofor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tofor/32/13727_2.png) [@tofor](https://boards.straightdope.com/u/tofor)\ **Post date:** [October 5, 2020, 2:01pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/3 "2020-10-05T14:01:01Z")
Snails and slugs have their breathing holes on one side only. > **[Pneumostome](https://en.wikipedia.org/wiki/Pneumostome)** > > Pages for logged out editors learn more > > The pneumostome or breathing pore is a respiratory opening of the external body anatomy of an air-breathing land slug or land snail. It is a part of the respiratory system of gastropods. > It is an opening in the right side of the mantle of a stylommatophoran snail or slug. Air enters through the pneumostome into the animal's single lung, the air-filled mantle cavity. Inside the mantle cavity the animal has a highly vascularized area of tissue that... ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [October 5, 2020, 2:03pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/4 "2020-10-05T14:03:04Z")
And there’s a Wiki article here. > **[List of animals featuring external asymmetry](https://en.wikipedia.org/wiki/List_of_animals_featuring_external_asymmetry)** > > Pages for logged out editors learn more > > > This is a list of animals that markedly feature external asymmetry in some form. They are exceptions to the general pattern of symmetry in biology. In particular, these animals do not exhibit bilateral symmetry which permits streamlining and is common in animals. > The crossbill has an unusual beak in which the upper and lower tips cross each other. Sponges are notable, lacking symmetry because they are sessile. The other examples in the article all seem to be minor asymmetric features on animals that are broadly symmetric. ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [October 5, 2020, 2:03pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/5 "2020-10-05T14:03:21Z")
Animals with radial symmetry - starfish, urchins, sea jellies… ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [October 5, 2020, 2:05pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/6 "2020-10-05T14:05:22Z")
I think those radially symmetric animals are also bilaterally symmetric. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [October 5, 2020, 2:12pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/7 "2020-10-05T14:12:11Z")
ETA: although I guess that’s only technically true, since there is no _unique_ plane of bilateral symmetry, and the development of radially symmteric animals is quite different. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [October 5, 2020, 2:20pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/8 "2020-10-05T14:20:40Z")
Not sure if this counts under the definition used. Flounder. They start out with eyes on either side of their head, but as they mature one eye migrates to the other side so they can still see while lying flat on the ocean floor. The other side of their body changes because they are lying on it, although I don’t know if there’s any change other than pigmentation on that side. ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [October 5, 2020, 3:58pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/9 "2020-10-05T15:58:22Z")
Just how do you divide an animal with five-fold radial symmetry into something bilateral? (Thinking of the starfish) ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [October 5, 2020, 4:36pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/10 "2020-10-05T16:36:19Z")
Since the arms are straight, a starfish has five planes of bilateral symmetry, running down the midpoint of each arm and extending through the midpoint of the gap between two arms on the other side. (If the arms were curved like a five-pointed “swastika” this would not be true, of course.) I’m only making a statement about superficial geometric symmetry. I realize that from the standpoint of evolution and development the radially symmetric animals are quite different from _Bilateria_. ETA: I’m rusty on evo-devo. Starfish _are_ in fact _Bilateria_. This appears to be a secondary return to radial symmetry. The larvae are not radially symmetric. ---
**Author:** ![Miller](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/miller/32/481_2.png) [@Miller](https://boards.straightdope.com/u/Miller)\ **Post date:** [October 5, 2020, 4:37pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/11 "2020-10-05T16:37:53Z")
Band saw? ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [October 5, 2020, 5:04pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/12 "2020-10-05T17:04:52Z")
Ok, here’s an evoloutionary overview. [http://tolweb.org/Animals/2374](http://tolweb.org/Animals/2374) We have _Porifera_ (sponges) and _Placozoa_ without any symmetry. Then _Cnidaria_ (jellyfish etc.) and _Ctenophora_ (comb jellies) with radial symmetry. Other animals fall under _Bilateria_, descended from a common ancestor that evolved bilateral symmetry. Within _Bilateria_ there are some interesting cases like the adult starfish that returned to a radially symmetric body plan, and molluscs with spiral shells. As well as countless examples of more minor deviations from bilateral symmetry. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [October 5, 2020, 5:19pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/13 "2020-10-05T17:19:57Z")
[Moties](https://en.wikipedia.org/wiki/The_Mote_in_God%27s_Eye#Moties) > **[The Mote in God's Eye | Moties](https://en.wikipedia.org/wiki/The_Mote_in_God%27s_Eye#Moties)** > > Moties are described as bipeds, about 1.3 meters (4 ft 3 in) tall, covered with fur whose color depends on the subspecies. Their most obvious feature is the asymmetric arrangement of arms, with two dexterous right arms and one heavily muscled left arm whose musculature attaches to the head, so that Moties have no left ear to match the large, membrane-like right ear. The backbone is jointed rather than flexible and the entire upper body swivels to turn the head. The face is simple and incapable of... (On the one hand, the OP didn’t say we couldn’t consider fictional species; on the other hand, the question wasn’t posed in Cafe Society; but on the [gripping hand](https://en.wikipedia.org/wiki/The_Gripping_Hand), Moties were the first such species to come to mind for me and are the classic example [in science fiction, anyway] of a species that does not have external bilateral symmetry.) 😉 ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [October 5, 2020, 5:22pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/14 "2020-10-05T17:22:27Z")
The Moties would presumably fall among the alien equivalent of _Bilateria_. A basically bilaterally symmetric body plan, with asymmetric features that are reminiscent of the fiddler crab’s large claw and the flounder’s eye arrangement. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [October 5, 2020, 5:24pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/15 "2020-10-05T17:24:58Z")
IIRC (because it’s been a long time since I read the books), the Moties, (including the various subspecies) were genetically engineered. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [October 5, 2020, 5:29pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/16 "2020-10-05T17:29:58Z")
There’s an open access review here on the genetic vs environmental basis for the development of asymmetric features among _Bilateria_. I haven’t had a chance to read it yet, posting in case anyone is interested. [https://royalsocietypublishing.org/doi/10.1098/rstb.2015.0417](https://royalsocietypublishing.org/doi/10.1098/rstb.2015.0417) ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [October 5, 2020, 5:58pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/17 "2020-10-05T17:58:42Z")
> [@Broomstick](#): > > Just how do you divide an animal with five-fold radial symmetry into something bilateral? (Thinking of the starfish) > [@Riemann](#): > > Since the arms are straight, a starfish has five planes of bilateral symmetry, running down the midpoint of each arm and extending through the midpoint of the gap between two arms on the other side. (If the arms were curved like a five-pointed “swastika” this would not be true, of course.) Or, C5v has 5σv, whereas C5 has none. ---
**Author:** ![wreidhead](https://avatars.discourse-cdn.com/v4/letter/w/e99b99/32.png) [@wreidhead](https://boards.straightdope.com/u/wreidhead)\ **Post date:** [October 5, 2020, 7:26pm UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/18 "2020-10-05T19:26:30Z")
Has no one mentioned humans? My left acorn hangs lower than my right one. (And causes me to trip all the time…) 😁 ---
**Author:** ![burpo\_the\_wonder\_mutt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/burpo_the_wonder_mutt/32/365_2.png) [@burpo\_the\_wonder\_mutt](https://boards.straightdope.com/u/burpo_the_wonder_mutt)\ **Post date:** [November 13, 2020, 2:10am UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/19 "2020-11-13T02:10:08Z")
How about owls? Even though they have two ears, one is higher and farther back on their head – helps with stereo location when they hunt. ---
**Author:** ![Jackknifed\_Juggernaut](https://avatars.discourse-cdn.com/v4/letter/j/9de053/32.png) [@Jackknifed\_Juggernaut](https://boards.straightdope.com/u/Jackknifed_Juggernaut)\ **Post date:** [November 13, 2020, 2:55am UTC](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259/20 "2020-11-13T02:55:01Z")
That seems pretty unique, and I think kind of a grey area. Most of the owl is symmetric, and ears are kind of like accessories. [Next page](https://boards.straightdope.com/t/which-animals-do-not-have-external-bilateral-symmetry/922259.md?page=2)