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Topics in Depth

The Theme of Controversial Cousins in Animal Evolution and Diversity

Relationships are hard. Four invertebrate phyla remain to be discussed and they are tricky. The phyla Phoronida, Bryozoaand Brachiopoda used to be grouped together into one group called lophophores. (A lophophore is a special type of feeding structure.)

More recent molecular evidence suggests that these phyla may have more in common with flatworms, rotifers, mollusks, and annelids, which do not have lophophores. These four all have the trochophore stage of life. (A trochophore is a little, swimming larvae that changes into the adult form.) That makes a big group that has either a lophophore or a trochophore to give us the mouthful of a name, Lophotrochozoa. This puts Nematoda and Arthropoda together as a new group called Ecdysozoa.

That still leaves one more invertebrate we haven't discussed, Echinodermata. These are deuterostomes, but are unlike the rest of the deuterostomes, which are all vertebrates.

Lost yet? Maybe a map would help.

Plenty of scientists make careers out of trying to figure this stuff out. In the meantime, we'll just describe the rest of the invertebrates, starting with the lophophores.

Phoronida, Bryozoa, and Brachiopoda have in one thing in common that no one else has, a lophophore. This is a ring or coil of tentacles that surrounds the mouth. These aren't like the tentacles we've seen before. These tentacles are hollow extensions of the coelom and have cilia on them. The anus is just outside the mouth, putting both ends of the digestive system at one end. Lovely mental picture, isn't it?

Phoronids are also called tube worms. You might see them if you take a submarine trip to the bottom of the ocean. They make their tube out of chitin, the same substance in arthropod exoskeletons. They attach themselves to the sea floor and stick their lophophore out of the tube to catch food particles in the water. If threatened, they pull their soft tentacles inside their tube.


Image from here.

Bryozoa means "moss animal." They live in clone colonies that share an exoskeleton. (The clones are called zooids, like the polyps that make up the Portuguese Man 'O War.) They use their lophophore to strain food particles out of the water like the tube worms. They have a lot in common with the Phoronids, except that they work cooperatively.

Brachiopoda look a little like clams, with two shells on the outside. Upon closer inspection of the shells, however, each half is symmetrical and but is different from the other half. Bivalves do not have symmetrical shells and the two shells do mirror each other. Brachiopods also hold onto the ground with a muscular foot called a pedicle while mollusks use a thread-like structure. And, of course, brachiopods have a lophophore and mollusks do not. Brachiopods are also called lamp shells.

Sea stars break a few rules. Rebels without a head. Echinodermata (sea stars, sands dollars, sea cucumbers) have radial symmetry and no head, despite being part of the larger bilateria branch of the animal family tree. Radial symmetry reappeared in the family tree, evolving from a bilateral ancestor.

This demonstrates an important point. Evolution is not a straight line and every common feature does not imply a common relationship. Ancestral echinoderms had bilateral or no symmetry, but somewhere along the way, having slightly more radial symmetry became advantageous and radial symmetry developed.

The radial symmetry of echinoderms is not like that of the radiata. It is usually in the form of five spokes. Echinoderms also differ in that they have an endoskeleton of plates and spines underneath the skin. This makes them look spiky on the outside, but without a shell.

Another unique feature of echinoderms is a water vascular system. This is a system of water filled vessels that runs from the center out, ending in tube feet that act like little suckers. The pressure in this system is what moves the animal. This is not a wimpy system. Sea stars can pull apart mollusks this way.

Check out tube feet in action and some pretty worried clams.

Echinoderms have male and females forms and fertilization is usually external. External fertilization is when the eggs and sperm meet outside the animal. In the case of sea stars, males and females release eggs and sperm into the water, where they meet up to make baby sea stars.

Echinoderms go through metamorphosis, starting out as a bilateral embryo that settles on the floor to grow. The embryo then develops into free-floating larva that grows into the adult form. It ends up on the sea floor again, but this time, it can move. Some echinoderms can also reproduce asexually by splitting themselves apart.

Lost an arm? No biggie, make a new one. Many echinoderms can regenerate parts and may even give them up on purpose to escape a predator. Check out the sea cucumber spilling its guts to save its skin.

Echinoderms range from filter feeders to grazers to hunters, but all live on the ocean floor. This is the largest phylum of animals to have only marine species.

Brain Snack

Giant, 11-foot tube worms living on underwater volcanoes. Need we say more?

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