ANNELIDA: ROUND, SEGMENTED WORMS
Christmas tree worm: Spirobranchus giganteus
All round, segmented worms are of the phylum Annelida. Annelids have bilateral symmetry and...
...two body openings: one for food, one for waste. This means they have a one-way digestive system. As an earthworm moves through the ground, it eats soil. The soil is stored in the crop, then ground into pieces by the gizzard. These pass into the intestines where they are broken down and absorbed by the blood. Waste is excreted through the worm’s anus. Annelids can have over 100 body segments, each of which (besides the end segments) has four setae. Setae are bristle-like structures used by worms to grip the soil and move. They also use two sets of muscles to move: one lengthwise and one circling the worm’s body. Unlike mollusks, worms have a closed circulatory system, so instead of simply flowing into all open areas, the blood only flows through blood vessels connected to the heart. They don’t have lungs and instead take air through their skin. A small brain is attached to their front segment. A main nerve cord is formed of nerves in each segment. This system allows the worm to sense light, heat, and moisture. This phylum reproduces sexually. All worms are hermaphrodites, but cannot fertilize their own eggs. Surprisingly, the most bountiful type of annelid in species is marine worms. Some species of marine worms look similar to Christmas trees. Many are free-swimming, but these are sessile, and have tentacles or tubes used to acquire nutrients.