From here on out, the animal body plan is bilaterally symmetrical. The remaining animals in the animal kingdom are collectively called the bilateria. This means that there is a front and back, top and bottom, and right and left. Bilateria also have a triploblastic embryo, which means that it forms three germ layers. ("Triplo-" means three and "blast" means a sprout or bud, so triploblastic means new growth.)
The bilateria can be grouped into three branches based on the kind of coelom formed in the embryo. As the embryo grows, it forms layers around the coelom. The acoelomates have three layers without a space ("a" means without, so this is without a coelom). The coelomates have a coelom made entirely from the middle germ layer, while the pseudocoelomates have a coelom made only partially from the middle layer.
AcoelomatesThe acoelomates include two phyla, Platyhelminthes and Nemertea (otherwise known as the flatworms and ribbon worms). These animals do not have an internal cavity filled with fluid and also do not have a circulatory system. They have flat bodies, making all cells close to the surface so they can access oxygen. They don't have the ability to move it about on their own. They do have true nerve and muscle tissue. See an example here.
Flatworms and ribbon worms have a head. This evolutionary development is called cephalization. This means that there is a true front and back end, with nervous system control and sensory organs concentrated in the head. Flatworms and ribbon worms have one or more ganglia, a collection of nerve cells that helps control nerve function in the organism. This is like a simpler brain. These worms also have sensory organs located in the head, another aspect of cephalization.
Brain SnackPlanaria are one kind of flatworm. They have light-sensing organs on their head called eyespots. It's a cute pet, for a worm.
Flatworms, phylum Platyhelminthes, are mostly marine animals. A few freshwater species splash around, too. Planarians, flukes, and tapeworms are all flatworms. All can reproduce sexually and asexually, though methods differ between them.
Some flatworms have found efficient, yet creepy, ways to find food. Tapeworms and flukes are parasites that inhabit a larger animal and use its resources. Blood flukes first inhabit snails and then move into the blood vessels in human intestines. Tapeworm heads are armed with suckers and hooks that lock its mouth into the digestive system of the host and take digested food directly from the host. Since they don't even have to digest their own food, they don't have a system for it. Best of all, tapeworms can grow to over 20 meters long inside a host.
The members of the phylum Nemerta are called ribbon worms or proboscis worms. Proboscis basically means a long appendage from the nose-like area. Unlike flatworms, ribbon worms have a small fluid-filed space near the front end. The worm uses water pressure to push out an extension or proboscis that is used to capture prey. Some things are better seen than described.
Some ribbon worms can grow up to 30 meters long. How's that for nightmare fuel?
The ribbon worms have a couple other things that flatworms don't: a complete digestive tract and a circulatory system. The ribbon worms do not have a heart, but use muscle contractions to move blood around their circulatory systems.
PseudocoelomatesThe next branch is the pseudocoelomates, which includes two phyla, Rotifera and Nematoda. The organisms that make up the Rotifera and Nematoda are called rotifers and nematodes. Most Rotifers live in fresh water and are very tiny, but still include a complete digestive tract and other organ systems. The pseudocoelom houses the internal organs. It also acts as a hydrostatic skeleton (that's also how jellies move). Nutrients and waste move through its fluid.
Rotifers get their name from a special circle of cilia around the mouth. (Rotifer means "wheel-bearer".) The cilia swirl water into the mouth, bringing in microorganisms to eat. See an example here.
Once again, truth is stranger than fiction when it comes to reproduction. Some species of rotifers are only female and only produce females from unfertilized eggs. Other rotifers produce two types of unfertilized eggs, which turn into females and a weird male that cannot even feed itself and only exists to produce sperm. The sperm fertilize some of the eggs, which turn into a special kind of zygote that can wait out a dry season until better times come around. When better times arrive, the rotifers will skip the fertilization part and females will make only females again.
Nematodes are the other pseudocoelomate. They are called roundworms, but are not the round earthworms we see in the yard. Earthworms have segments that look like little ridges on the outside. All the worms we've discussed so far are smooth.
Roundworms are everywhere: in salt and fresh water, in plant tissues, in the soil, and in animal tissues. They are even in our puppies.
They can be found from arctic areas to deserts and everywhere in between. There are thousands of known kinds and probably many others that haven't been discovered. They have a similar digestive system and hydrostatic skeleton to the rotifers, but have a tough covering called a cuticle. For a surprising turn in events, reproduction in these animals is usually sexual with separate female and male forms.
Nematodes play a tremendous role in the ecosystem. They are major players in decomposing organisms; they recycle nutrients needed by other organisms. Nematodes are also major pests for humans. They attack the roots of the plants we eat. Nematodes also parasitize humans. Some lovely examples include hookworm and Trichinella. Hookworm lives in the small intestines and lives off the host's blood. Trichinella, which starts off in the intestine, end up burrowing into the muscle and eventually into the heart.
Some nematodes can be kind of pretty.
To sum up this group:
- We now have a clear front and back.
- Cephalization is the norm, with nervous control and sensory organs centralized in the head.
- Embryos are triploblastic.
- We begin to see organs.