Simple Invertebrates Sponges, jellyfishes and coral, flatworms, roundworms, and segmented worms too!

So what do you know about simple invertebrates? Do you know the main characteristics of each of the five phyla? Let’s check them out.

Phylum Porifera (Sponges) Simplest animals of all (some of the oldest too). All are aquatic (freshwater and marine). Only two layers of cells, but no tissues or organs! Their skeletons are made of tiny spines called spicules. Cannot move (grow attached to the bottom). Mostly filter feeders. Uses?

The mighty sponge!

Phylum Cnidaria (aka Coelenterata): Jellyfishes and Corals More advanced than sponges. (for example, they have tissues, but still no organs). Their body is a hollow cavity with only one opening (for food AND wastes)! Entirely aquatic (freshwater and marine). All have stinging cells, but not all can harm people. Some are pretty nasty predators. Others filter-feed on small particles and organisms in the water.

Medusae vs. Polyps MEDUSAE Ex. Jellyfishes. A medusa is free-swimming. Most are solitary. Their opening is on the bottom of their body. POLYPS Ex. Corals Polyps are like an upside down medusa. Their opening is on the top of their body. Coral polyps are tiny and live in huge colonies of thousands. Corals build their “home” as the colony grows larger and larger.

Jellyfish Most jellyfish are harmless to people, although not so to small fish and other marine organims.

Portuguese man-of-war. Look out!

Corals: beautiful colonies of polyps.

More Cnidarians: hydra and sea anemones. Is it a medusa or a polyp?

Not to confuse you, but did you know that jellyfish start their lives as polyps, but become medusae as an adult? It’s true! Here is a jellyfish life cycle.

Phylum Platyhelminthes (Flatworms) Even more complex. Long, flat body. Clearly defined head with mouth. Some have senses, a few even have eyespots. Some move freely. Many are parasites (ex. Tapeworm). Some live in the water (fresh or marine), others live inside their host!

Flatworms

Phylum Nematoda (Roundworms) Another step up the ladder! Some even have a simple nervous system. Some have a digestive system that runs through the entire body -- with two openings, one for food to enter and one for wastes to leave. Most are parasites. Many are harmless microscopic critters that live in the soil. Also, the trichina worm (in pork), as well as hookworms and heartworms (common in pets).

Roundworms! One teaspoon of garden soil may contain as many as 10,000 tiny nematodes (roundworms) in it!

Phylum Annelida (Segmented Worms) Most complex worms of all. Tube-like body divided into segments. Sophisticated digestive system! (mouth, crop and gizzard, intestine, anus) Many other specialized organs heart, blood vessels, nerve cords, simple brains) Can move well. Some aquatic (leeches, clam worms), some terrestrial (earthworms). Some are parasites (leeches), others are very important decomposers (earthworms). Any other uses for annelids?

Phylum Annelida (Segmented Worms)

So NOW what do you know about the simple invertebrates? Do you know the names of the five phyla of simple invertebrates? Do you know the main characteristics of each of the five phyla? Can you identify examples of animals in each of the five phyla?