Respiratory and Circulatory Systems

All annelids live in moist environments; in fact, most of them are actually aquatic.  Even the earthworm can survive only in damp soil.  This means that the body surface, or parts of it, can still serve as the site of gas exchange.  However, because annelids are considerably larger, more active and more complex than the phyla already considered, some means of transporting gases, wastes and nutrients through the body, other than simple diffusion is required. 

            The function of internal transport in annelids is performed by the coelomic fluid which is moved around by contractions of the body wall musculature and, in the majority of segmented worms, by a true blood vascular system.  To enhance the capacity of the blood or coelomic fluid to carry oxygen, these fluids may contain respiratory pigments.  These pigments easily combine with oxygen and increase the capacity of blood to transport oxygen.  When oxygen concentration is low the pigments release oxygen into the respiratory system.  In the annelids the respiratory pigment is hemoglobin which may either be contained within cells or be in solution in the blood.

a)      Place an intact, live Lumbricus sp. dorsal side up on a dissecting pan.  Note the dark pulsating line on the dorsal surface, the dorsal blood vessel.  This is the main pump of the worm's circulatory system.  Return the worm once you've made your observations and continue the examination of the circulatory system on the preserved worm.

b)      Locate the dorsal blood vessel on the preserved specimen and trace it toward the mouth. There are five pairs of pseudohearts surrounding the esophagus (Figure 2). The pseudohearts look like black rings on the dissected specimen. The blood in the dorsal vessel flows anteriorly.  It is pumped downward by five pairs of  the pseudohearts to the ventral blood  vessel. You will observe the ventral blood vessel later. The blood in the ventral blood vessel flows posteriorly.  Small vessels in each segment carry blood from the ventral vessel back to the dorsal vessel  (Figure 4).  There are also blood vessels going to the major organs including the digestive tract and excretory organs.

Figure 4: Diagrams of circulation in the earthworm.

Dorsal blood vessel

Pseudohearts