I will attempt to explain, in detail, why this is so. Then, just for fun and to stretch my brain (and yors), I will attempt to make a terrestrial echinoderm while still keeping the rules of basic echinoderm anatomy and biology in mind. I will cheat (OMG! You're not playing by the rules!), butnot when it comes to biology (damn it). Instead, I will chea at the very definition of terrestrial. You'll see soon enough. But first, let's define. What exactly is an echinoderm?
WHAT IS AN ECHINODERM:
Echinoderms are members of the phylum Echinodermata. The phylum name is derived from the Greek word "echinoderma", which means "spiny skin". The phylum contains about 7,000 living species, making it the largest phlyum with no freshwater or terrestrial members. After the chordates, it is the second largest grouping of deuterostomes (a deuterostome is a creature in which the first opening on the blastula (a ball of cells that have just begun to differentiate) forms the anus rather than the mouth in protostomes).
There are two main subdivisions of echinoderata. One, Pelmatazoa, consists of the crinoids and the extinct paracrinoids, otherwise called sea lilies. The other, Eleutherozoa, consists of nearly all species of echinoderms, such as the asteroids (starfish), echinoids (sea urchins and sand dollars), holothuroids (sea cucumbers), and ophiuroids (brittle stars). For this blogpost, we shall concern ourselves with the eleutherozoans and their basic, and somewhat restrictive, anatomy.
ANATOMY OF ECHINODERMS:
As we've already established, much of the echinoderm diversity is contained in eleutherozoa. Most of the eleutherozoands are motile, and it is thee that we will also focus on. Conveniently, these are also the echinoderms most familiar to us. They possess the defining characteristics of echinoderms. This includes radial symmetry, tough outer skin, and a water vascular system. Let's start with the tough outer skin.
The skin, while hard, really owes its success to the calcerous skeleton, located in the mesoderm (the middle germ layer). The skeleton is composed of calcerous plates, held together by the skin. Despite the robustness of these plates, fossil echinoderms are rare because, once the skin rots away, the plates disarticulate, not holdig their shape.
Moving o to something a bit more interesting. Radial symmetry. It all starts with the larval development. All echinoderms are born ciliated (possessing cilia) and look quite similar to chordate larvae. As the echinoderms grow, the left side continues to grow whereas all growth on the right becomes absorbed into the left side, thus producing the radial symmetry present in all echinoderms.
"Whoa whoa whoa," you're probably saying. "Back up. The holothuroids [sea cucumbers if you don't remember] are bilaterally symmetrical. How is radial symmetry a defining characteristic for all echinodermms?"
Actually, the holothuroids are radial symmetrical. They've just evolved secondary bilateral symmetry. If you take a cross section of a sea cucumber, you'll see the classic radial symmetry. The large outer spines are arranged in a radial symmetrical patter, as are the tentacles on the head and the teeth.
Interestingly, ALL echinoderms, again without variation, are pentaradial. Basically, this means that the organism can be rotated in any multiple of five ways though the limit to this depends on how many limbs the organism has) and still look the same. Most echinoderms (most notably starfish) can only be rotated five ways. However, some (like the crinoids) can be rotated 200 ways and maintain symmetry.
As is well known, ehinoderms can regenerate lost body parts. Sea cucumbers have been known to discard whole organs when in danger, then regrow them. It is commonly known that starfish can regenerate lost limbs, but lost limbs can also regenerate whole starfish. In fact, some starfish have even been known to colonize whole areas SOLELY through asexual reproduction
Let's look at asteroid (starfish) anatomy so we can see why this is so. Asteroid anatomy is, in my personal opinion, the easiest eleutherozoan anatomy to understand. Each arm, arranged in a pentaradial pattern around the main body (which contains a pyloric, or main, stomach), contains "veins" (channels where water is pumped throughout the body), a pair of gonads, and ampulae, which are attached to a radial and lateral canal allowing the creature to move. I'd like to focus on the gonads for now, but we'll give the water vascular system its own section.
I'd like to posit that the gonad pair in each arm is the reason the asteroids and other echinoderms are able to regenerate so easily. With a ready store of reproductive material at hand (no pun intended), it seems like they shoud be able to regenerate relatively easily. And they are.
I'm not sure how accurate this theory is, but it seems quite accurate in my eyes.
Next up: an entire section devoted to the water vascular system. The most important aspect concerning the plausibility of terrestrial echinoderms. And the most defining characteristic all echinoderms possess.
THE WATER VASCULAR SYSTEM:
The water vascular system is a system completely unique to echinoderms and is thus a defining characteristic of the phylum. It is used by echinoderms for respiration, locomotion, digestion, and circulation.
The echinoderms have evolved specialized muscles around the "veins" that push the water into the tube feet. The force of the water causes the feet to extend and push against the ground. Then the tube foot relaxes, allowing the foot to retract. This process, completed in many tube feet at the same time, produces their movement. It is kind of slow going, which is why most documentaries that featur echinoderms in motion use time lapse photography. But it does work well in the water,wehre the supply to the "veins" is constant.
While this system does circumvent the problem of having to use gills quite effectively, it does have its disadvantages.
Without a constant supply f water, there is no way the creature could move. They have evolved a hole called a medroporite that, through the use of the stone canal (a canal attached to the medroporite), pumps waer into the ring canal, which transports the water all over the body to be used. And it is not just locomotion that would be affected by becoming terrestrial. Pretty much every aspect of echinoderm anatomy would be adversely affected, from repiration to digestion. The greature would begin to shut down moments after it left the water. The skin would dry out and the creatue would die from either dehydration or predation (echinoderms are slow movers after all).
But there is one way to make a terrestrial echinoderm while still keeping the rules of biology in mind. The only rules you have to ignore are the rules of English. Specifically the dictionary.
A PLAUSIBLE "TERRESTRIAL" ECHINODERM:
The dictionary defines terrestrial as "belonging to the land rather than the sea or air; living or growing on land rather than in the sea or the air". It is this definition that I will ignor. It does owrk in defining what a terrestrial animal is, but completely terrestrial echinoderms are nearly impossible.
What I suggest is an amphibious existence. Though it will, by necessity, have to spend more time in water than on land.
The species I've thought up has the scientific name Balbidoris ostracoeides, meaning "clam water-valve", so named because of a unique anatomical addition and its extensive diet of clams. In fact, clams are the very reason B. ostracoiedes (to save my fingers, I'll just call it the black balbidor, so named because of the black coloration of the arms) ever came onto land in the first place. The anatomical addition, called a balborite, is a large valve that can clog up the medroporite when on land. The balborite is located in the stone canal. When not in use, it is deflated like a balloon but can become inflated when water surges into it from the "veins". This blocks the medroporite and the creature, in effect, holds its breath.
The black balbidor crawls around on rocks sitting close to the ocean where clams are often located at low tide. Once it gets close to the clam, specialized tube feet make use of the natural grooves in the clam shell so that it can open the area between the shells just enough to insert its stomach ino the clam. This is similar to the way modern asteroids (starfish) do it today.
The black balbidor is actually not a descendent of the asteroids, but rather of the ophiuroids (brittle stars). In ophiuroids, the medroporite is located in one of the jaw plates rather than on the upper side of the body like in asteroids. This will make evolution of a holding-the-"breath" method easier. Plus, instead of using tube feet for locomotion, the ophiuroids use their arms directly, which conserves water, critical to a land echinoderm. Also, ophiuroids can tolerate brackish water, which is the closest any echinoderm has gotten to fresh water and thus land life. But I don't see that tolerance evolving any further.
I'd expect the black balbidor (or something similar) to evolve in 150 to 200 million years. If it did at all. With so many serious competitors, such as other fish, cephalopods, gastropods, mammals, birds, reptiles, and amphibians, land echinoderms don't stand a chance at gaining dominance on land, much less evolving to it. Now, a low tide land walker is, while still not the most plausible idea in the world, is probably the closest to land life echinoderms will get.
Picture of the black balbidor:
Land echinoderms. Whoever first thought of the idea was pretty imaginative. Perhaps too imaginative. It's an interesting concept to say the least. However, it's not a very plausible (or even possible) one.
There are more plausible alternatives. Try other fish or cephalopods or a diversification of gastropods if you want plausible new land life. But I'd suggest you stick with what Mother Nature has already given you: mammals, birds, reptiles, amphibians, and arthropods. It may not be the most exciting or imaginative thing in the world, but look at the world around you. There are plenty of creatures that no speculative biologist could imagine living that way. Like beetles that breath with bubble aqua-lungs. Or birds that have a tongue stored in a cavity that is wrapped around their brain. Or even a human.
Things of interest can be made without resorting to the impossible. The more plausible you make your fuure evolution project (which means you shouldn't include land echinoderms), the more likable it is. Projects that have impossibilities or even implausibilities have a tendency to get ignored. Even if it follow the rules of cool rather than the rules of evolution.
However, if you understand the rules of evolution, you're guaranteed to come up with a great future evolution project. That means no land echinoderms. However, it will still be a nice little world kiddies. If you can make it.