LOPHOTROCHOZOA: PHYLUM ANNELIDA
Members of the phylum Annelida (segmented worms), such as earthworms, leeches, and polychaetes, are triploblastic, bilaterally symmetrical, and eucoelomate. They have a complete digestive tract, a nervous system showing cephalization, a closed circulatory system, and an excretory system (based on nephridia) for removing wastes. They also exhibit metamerism, the division of the body into segments. Segmentation is advantageous during development, where greater efficiency is obtained by constructing a whole organism out of similar subunits. In the adult, locomotor activity is enhanced because of the independent nature of each segment and the flexibility afforded by a series of segmented parts. Segmentation also gives these phyla a survival advantage: since segments are generally similar in form and function, damage to one or several segments does not necessarily compromise body functions. In addition, annelids exhibit a body wall with both longitudinal and circular muscle layers, which, along with segmentation, allows these animals to be quite mobile. The majority of annelids possess chitinous bristles, called setae or chaetae, which assist in gaining traction for locomotion or in anchoring the animal to the walls of its burrow.
There are three major classes within the phylum Annelida:
Class Polychaeta - mostly marine worms, such as Nereis sp. (clamworm)
Class Hirudinea - the leeches (mainly freshwater), such as Hirudo sp.
Class Oligochaeta - mostly freshwater and terrestrial worms, such as Lumbricus sp. (earthworm)
In this exercise, we will examine the external and internal structure of a representative oligochaete, Lumbricus sp. Living oligochaetes will be available for observation, as directed by your instructor. In addition, preserved polychaetes (e.g., Nereis sp.) and leeches (e.g., Hirudo sp.) are available as demonstration specimens.
Obtain an earthworm from the supply and place the animal in a dissecting tray. You should use a dissecting scope to fully appreciate the external anatomy. The most obvious external feature is the clitellum, a swollen area in the anterior third of the specimen. This region functions in reproduction by secreting mucous which holds the participants together during sperm exchange.
Orient the worm dorso-ventrally by locating the setae (= chaetae). Run your fingers along the animal to feel the rough texture produced by the setae. Four of these structures are found on the ventral surface of each segment. They provide traction during locomotion. Using the clitellum as a landmark, identify the anterior and posterior ends of the worm. The anterior end has a mouth opening, covered by a lobe termed the prostomium. The posterior segment bears the anus. Starting with the segment that holds the mouth, locate segment 14. If you look carefully, you can see the openings for the oviducts on the ventral surface of segment 14 and the sperm ducts on the ventral surface of segment 15.
Pin your worm, dorsal side up, near one edge of a dissecting pan. Using fine scissors or a sharp scalpel, make a shallow longitudinal cut along the dorsal surface of the worm. (Be sure not to cut into the dorsal vessel and the intestinal tract, which are located just below the body wall.) Note that the body wall is held in place by septa (internal divisions between metameres). Cut the septa along the length of the worm on both sides of the intestine. These septa divide the coelom into separate cavities. Pin the body wall to the bottom of the pan, inclining the pins away from the worm. Keep your preparation moist at all times.
Observe the digestive tract running from mouth to anus. Just posterior to the mouth opening is the muscular pharynx. The torn muscles associated with the pharynx were attached to the body wall. When these muscles contract, food particles are sucked into the mouth. The esophagus is posterior to the pharynx and is surrounded by cream-colored bodies that will be studied later. The esophagus expands into a thin-walled storage structure, the crop. Probe the wall of the crop gently and note its texture. Just posterior to the crop is the muscular gizzard. This is a grinding structure with thick, muscular walls (obvious by gentle probing). Food is passed from the gizzard to the intestine, where further digestion and absorption occur. The intestine ends at the anus.
Earthworms are monoecious, with cross-fertilization the rule. The gonads (testes and ovaries) may be too small to see, but many of the associated reproductive structures are clearly visible. Note the large, cream-colored structures associated with segments 9-12. These are seminal vesicles. Testes are associated with seminal vesicles. Sperm are passed from the testes to the seminal vesicles for storage prior to copulation. During copulation, sperm exit through an opening at segment 15. The ovaries are located in segment 13, and their duct system opens to the outside in segment 14. Two pairs of small, round, cream-colored seminal receptacles on the ventrolateral body wall in segments 9 and 10 receive sperm during copulation. During copulation, two worms line up facing in opposite directions, segments 9 and 10 of each opposite the clitellum of the other. The anterior halves of the worms are wrapped in a mucous sheath. Sperm from the seminal vesicle of each passes into the seminal receptacle of the partner. When worms separate, sperm transfer has been mutual. Each worm then forms a second mucous sheath at the clitellum. Eggs are discharged into the sheath and the worm begins backing out of the sheath.
Circulation in the earthworm is through a series of closed vessels. The two main vessels that can be seen in your dissection are the dorsal and ventral blood vessels, which also serve as the main pumping structures. The dorsal vessel is the dark line running along the dorsal surface of the digestive tract; it carries blood anteriorly. In the posterior third of your worm, carefully cut through and remove about three centimeters of the digestive tract. The ventral blood vessel can usually be seen adhering to the segment of intestine removed. In the ventral vessel, blood moves posteriorly. Segmental branches off the ventral vessel supply the intestine and body wall with blood. These branches eventually break into capillary beds to pick up or release nutrients and oxygen. Gas exchange occurs between the capillary beds of the body surface and the environment. Oxygen is carried by the respiratory pigment hemoglobin, which is dissolved in the fluid portion of the blood. From these capillary beds, blood is collected into larger vessels that eventually unite with the dorsal vessel. At the level of the esophagus, segmental branches are expanded into five pairs of aortic arches, or what have been called "hearts". They are dark, expanded structures on either side of the esophagus. Although these are contractile, they only function in pumping blood from the dorsal to the ventral vessels.
Using the lowest power of a compound microscope, examine a cross-section of Lumbricus. Note the noncellular, protective cuticle that covers the epidermis. Just below the epidermis, note the circular muscle and, below that, the longitudinal muscle. The mesodermal lining of the inner body wall is the peritoneum, and the coelom is the space between the peritoneum and the gut tube. Covering the dorsal vessel and the intestine are cells called chlorogogue cells. These are involved with glycogen and fat synthesis and urea formation. Folding to the interior of the intestine is the typhlosole, which increases surface area for secretion and absorption. Two lateral neural blood vessels and one subneural blood vessel are associated with the ventral nerve cord. Depending on the section studied you may also see nephridia and setae.
Here is a student video of an earthworm dissection.