3. Planarians (Phylum platyhelminthes).

Planarians are flatworms. They are common in streams in our area. They are smal l, usually dark brown or black, arro w-shaped organisms that glide along the undersurface of rocks by means of cilia on their lower surfaces. They have an esophagus and mouth on the middle of their undersides. They capture and consume small aquatic organisms.

4. Segmented Worms and Leeches (Phylum annelida).

You are familiar with earthworms. You may find earthworms near the edges of st reams or ponds. There ar e also aquatic species, including many marine forms. They typically have a long worm-like body which is divided into segments (block-like sections). The marine forms often have broad paddle-like extensions on each segment to help them absor b oxygen. Leeches are annelids that are external parasites. They attach to fish or other organisms and bite through the outer layers of cells to absorb blood an d fluids from the host organism.

5. Snails, Clams, Oysters (Phylum mollusca).

Many of the organisms of this group have distinctive hard calcareous shells whi ch cover and protect them. So me are sessile; others move slowly along surfaces or burrow into the bottom subs trate. Some mollusks such as slugs, octopus and squid do not have externa l shells. Note the diffident kinds of shells on the mollusks you observe on the field trips.

6. Insects and Crustraceans (Phylum arthropoda).

Organisms in this group have jointed legs and other appendages and hard jointed other skeletons called e xoskeletons. In freshwater systems, two of the most common arthropods are insect s and crustaceans. Insects typically have six legs although some of the larval f orms don't have any legs. Think of the caterpillars (larvae of butterflies), for example. Many insects are aquatic in their juvenile stages (larvae or nymphs), but emerge to form winged adults that can fly. Some insects continue to live in wate r as adults. You should see many different kinds of insect larvae on your field trips. We have some keys to help you identify these organisms.