As a consequence, the overall dynamic of their life mode is different from an animal with bilateral symmetry. No face, no "front" or "back". Thus, the way food moves through the body is different depending on where the mouth is located and consequently, where the anus is located!!
Since each living class of echinoderms has a fairly unique shape and body morphology, each group has a bunch of unusual specializations that function to facilitate the expulsion of poop!
To be honest, not every member of each class below has what is shown below. These are unusually prominent examples..but it still begs the question "What is it used for?" How many other animals have so many unusual anal adpations??
5. Crinoids: Anal Chimneys & Pyramids
So, crinoids are suspension feeders. They almost kind of resemble plants. Most living ones are known as "feather stars" and are basically cups with arms for filtering water. But earlier forms of crinoids are known as "stalked crinoids" which have a stalk. I had a gallery of the older Paleozoic ones here a few weeks ago..
Here's a living one from the recent Okeanos Expedition to the Marianas region to give you an idea of what they look like..
So, the mouth in stalked crinoids is inside that cup at the base of where all those arms converge. The anus for these animals is ALSO in the cup. So, there's likely a strategy/adaptation for these animals to push the poop/excreta OUT of the anus, such at it does not end up getting "re-eaten" by the mouth...
According to Fossil Crinoids by Hess et al. there were several fossil (Paleozoic) forms which had fairly straightforward strategies for dealing with ensuring that poop was discharged FAR from the mouth..
For example, here is the Mississippian Uperocrinus nashvillae with an explanatory diagram from Fig. 37 of Hess et al's book. Basically that huge pointed structure on top? That's called the anal tube (or sometimes in other animals.. an anal pyramid or even an anal chimney!)
These extended structures serve to project the anus (and the excreted poop) well AWAY from the mouth (the feeding arms would come off right at that wide "ledge" around the center of the specimen..
The tube allowed waste products to be whisked away far from the mouth of the crinoid, which was at the base of the arms. Some researchers suggest that the long tube served another function as well: it may have helped stabilize and direct the filter-feeding fan of outstretched arms in a stiff current, something like the tail of an airplane or a panel on a weather vane.
One last weird crinoid is this one: Bicidiocrinus wetherbyi
Another Mississippian (i.e Paleozoic) stalked crinoid.. and this is kinda weird. So, there's the cup and the arms and that cone is the anal cone (=tube, pyramid, chimney, etc.) BUT it also has this weird additional protective "spiniferous canopy" around it!!
The diagram on the right shows this fully "reconstructed"..
One LAST MINUTE ADDITION: David Clark (@clarkeocrinus) provides this ASTONISHING Proteriocrinus with a very considerable anal chimney!! which looks to extend nearly the length of the cup and arms!
@echinoblog A Poteriocrinus from the Dev Arkona Fm in Ont of the John & Mike Topor collection. Quite a chimney! pic.twitter.com/gHG76PiLR5— David Clark (@Clarkeocrinus) April 26, 2016
Probably one of the best known but most poorly recognized of the various echinoderm anal structures is the ANAL SAC in diadematid (diadematoid?) sea urchins.
This includes Diadema, Astropyga, Echinothrix and all of urchins in this family. Usually these are tropical and characterized by long, sharp spines AND a very distinctive "anal sac" present on the TOP of the body.
The problem is that many people see this big eye-shaped ball on the top of the sea urchin body and assume that it is an eye of some kind...
I actually explained in great detail here how this was actually the unusual ANAL SAC which is characteristic of this type of sea urchin for an error in New Scientist.
That said, they DID publish a WONDERFUL picture by David Fleetham of Astropyga radiata venting poop OUT of the anal sac!!
Basically, this is a transparent or translucent bulb or sac extension from the anus through which feces passes on its way out of the body.
See those little round things that look like corn kernals? Sea urchin poop!
But its not JUST shallow-water diadematid urchins that have this anal sac. Here's a deep-water Aspidodiadema from the recent Hawaiian Okeanos expedition. I talk a bit about these here
These were videos of these animals from 2000-3000 METERS below the surface. Aspidodiadematid urchins are classified in the same general group as diadematids. and they too seem to have this anal sac or cone...
2. Anal (or Epiproctal) Cone
Sea Stars/Starfish in a larger group called the Paxillosida (the mud and/or sand stars) have a specialized structure which sits right on the center of the disk called the anal or epiproctal cone.
A brief anatomical note- Although historically called an "anal cone" these starfish don't actually have a complete gut and so, the opening on the disk center is not actually the anus since it doesn't connect with the interestine. Hence the name "epiproctal": EPI is Greek for "upon" and PROCT is Greek for "anus".. hence the cone or structure UPON the anus..
|From (Fig. 2 from Shick 1976)|
Observations of Ctenodiscus under hypoxic conditions led to the illustration above. Basically, its thought that the cone gets more enlarged as hypoxia and hydrogen sulfide increases. The extension of the cone extends through the surface, with the tip at the surface. For your typical 6.0 cm diameter animal, these animals can have a cone that can attain 3 to 4 cm and extend 2 to 3 cm above the mud. It can leave this extended for over an hour. As the picture suggests, it can move around and push through sediment as the mud shifts, and etc. So, it can move around.
This also serves to make the top surface of the animal thinner, allowing easier gas exchange and opening up a channel to the surface water above the sediment surface!!
|From Shick 1976 Marine Biology: http://link.springer.com/article/10.1007/BF00387613#page-1|
|from CSU Fullerton: http://biology.fullerton.edu/biol317/ftm/ft_s15_cat_4_25_15.html|
And finally, one of the best known of echinoderm anal defenses: the anal teeth in sea cucumbers!
I've reported on these before. There's at least one interpretation that these structures are defensive in nature and work to keep pearlfishes (and likely other commensals or parasites) from inhabiting the cloaca.
There's quite a few crabs and shrimps that live in and around sea cucumber anuses. See more here.