Primitive

Lancelet: Not a Fish
Lancelets (Amphioxus) are "fish-like" creatures that split from vertebrates 520 million years ago. They have no fins, no bones, no blood, no heart, no respiratory system, no eyes, and no brain. They represent a anomalous holdover from the ancient period of life before the Cambrian explosion that established modern body patterns.

The lancelet has inspired a song, set to the tune of "It's a Long Way to Tipperary"

''It's a long way from Amphioxus. It's a long way to us. ''It's a long way from Amphioxus to the meanest human cuss. ''Well, it's goodbye to fins and gill slits, and it's welcome lungs and hair! It's a long, long way from Amphioxus, but we all came from there.''

Wikipedia entry: http://en.wikipedia.org/wiki/Lancelet

Tunicates: Ancient Chordates
Tunicates are sessile organisms that attach to rocks in the ocean.

What is interesting about them is they are "chordates"--precursors of vertebrates. Their larva, which freely move through the ocean, have notochords--primitive spinal cords.

Tunicates are more closely related to vertebrates than they are to invertebrates, but look like a kind of coral. This makes them like distant cousins to humans.

Wikipedia entry: http://en.wikipedia.org/wiki/Tunicate

Slime Molds: Emergent Intelligence From a Bag of Amoeba
Slime molds are social amoeba that form colonies.

Scientists have coaxed them to act as biological computers: solving mazes, learning from past events, optimizing their physical configuration, and act as logic gates.

Pretty sophisticated for a bag of amoeba.

Wikipedia entry: http://en.wikipedia.org/wiki/Physarum_polycephalum

Trichoplax: Simplest Model Animal
Placozoa are represented by a single species (Trichoplax adhaerens) that is considered to be the simplest invertebrate animal.

It was discovered in 1883 oozing around an aquarium in Graz, Austria.

It has a few thousand cells in three layers, and has no back or front.

Wikipedia entry: http://en.wikipedia.org/wiki/Trichoplax

Loricifera: Only Animals That Live Oxygen-Free
Loricifera are a recently-discovered phylum of life. Certain species are the only animals capable of living without oxygen.

From Wikipedia: "Three species of Loricifera have been found in the sediments at the bottom of the L'Atalante basin in Mediterranean Sea, more than 3,000 meters down, the first multicellular organisms known to spend their entire lives in an oxygen-free environment. They are able to do this because they rely on hydrogenosomes (or similar organelles) instead of on mitochondria for energy."

Wikipedia entry: http://en.wikipedia.org/wiki/Loricifera

Boneworms: Making a Living on Whale Bones
Boneworms were discovered in 2002 consuming the bones of whales lying on the seafloor. They have no mouth or stomach, but rely upon acid and specialized bacteria to digest bones.

All boneworms are female, except for microscopic males that live inside the bodies of the females.

Wikipedia entry: http://en.wikipedia.org/wiki/Osedax

Xenoturbella: What Is It?
Xenoturbella is a primitive animal that has no brain, no gut, no gonads.

In fact it has no internal organs at all.

It is thought to represent a base form of creature that predates the development of internal organs.

Wikipedia entry: http://en.wikipedia.org/wiki/Xenoturbella

Ice Worms
Ice worms travel through glacial ice without freezing to death. TODO

Blue Dragon Sea Slug
The Blue Dragon Sea Slug is a free-swimming sea slug with a unique body form. It floats upside down in the water.

It feeds upon jellyfish, incorporating their stinging nematocysts into their own bodies for protection.

Wikipedia entry: http://en.wikipedia.org/wiki/Glaucus_atlanticus

Nautilus: Living Fossil
The nautilus is a living fossil that is an ancestor of the octopus, yet retains the shell of its mollusk ancestors. The shell contains chambers that it uses to regulate its buoyancy in the water.

The nautilus is highly intelligent, and is known to hypnotize its prey with a display of colors prior to attacking.

Wikipedia entry: http://en.wikipedia.org/wiki/Nautilus