BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. UTILIZATION OF ENERGY

9.4. Gas Exchange

9.4.3. Annelids, e.g., the Earthworm

The earthworm belongs to the phylum Annelida. It lacks any specialized Organs dedicated specifically to gas exchange; instead, Respiration occurs via diffusion across the entire body surface. Specialized organs are essentially unnecessary for them because their cylindrical body shape provides a high surface area-to-volume ratio, and their relatively low activity levels mean they consume modest amounts of oxygen.

Nevertheless, unlike certain simpler animals and unicellular organisms, Annelids possess a Circulatory system, with the respiratory pigment Hemoglobin dissolved in their Blood. Contractions of major Blood Vessels propel the blood, along with the dissolved gases, throughout the body; this process also helps maintain steep diffusion gradients.

The earthworm's thin Skin (cuticle) is kept constantly moist by secretions from epidermal glands. Capillaries are situated within the epithelium directly beneath the cuticle (Fig. 9.14). The minimal distance between the blood vessels and the body surface ensures rapid diffusion of oxygen into the blood. Earthworms have virtually no protection against desiccation and therefore tend to inhabit exclusively moist environments.