Phylum Annelida
Canadian Campbell 2nd ed  Pages 742 - 745)

The annelids are segmented worms and consist of three classes:

1. Class Oligochaeta: Earthworms and a variety of freshwater species


2. Class Polychaeta: All marine forms such as Nereis and Arenicola


3. Class Hirudinea: Leeches or blood suckers

Class Oligochaeta

Examine cross sections of the earthworm Lumbricus taken through the intestinal region:

• Whole Section x10
• Ventral Portion of Section x40
• Dorsal Portion of Section x40

In the sections note the following features. The body wall is composed of four layers. The outermost layer is the thin, noncellular cuticle secreted by the epidermis, which consists of a layer of columnar epithelial cells, among which are many glandular cells. Beneath the epidermis is a layer of circular muscle fibers. The next and thickest layer of the body wall is made up of longitudinal muscle fibers arranged in a feather-like pattern. The innermost layer of the body wall consists of a thin layer of flattened cells composing the peritoneum. The coelom (image) is the space between the body wall and the digestive cavity. In the coelomic cavity, locate the dorsal and ventral blood vessels and the solid ventral nerve cord.

The thin-walled intestine bulges laterally in each segment; its dorsal wall has an infolded typhlosole. The wall of the intestine is composed of three layers. The outermost layer is made up of slender columnar cells containing numerous granules and chloragogue cells (mesodermal origin). You should make a sketch of the above structures.

Consider the following questions:

• What is the body cavity called?
• How does this cavity differ from that found in the nematodes?
• What advances does the earthworm digestive system show over the digestive system of planaria?
• How is internal transport of nutrients and wastes to and from the cells accomplished in earthworms?
• How did this occur in planaria?

The earthworm also illustrates segmentation.

Examine sketch of a worm and note the segmented body plan