Of the three major phyla of protostomes, only the arthropods and annelids 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 identical modules. In the adult, locomotor activity is enhanced because of the independent nature of each segment and the flexibility afforded by a series of segmented pans. Segmentation also gives these phyla a survival advantage. Since many segments are similar to other segments in form and function, damage to one or several segments does not necessarily compromise body functions.
The phylum Annelida (segmented worms) is diverse, containing the earthworms, leeches, and marine worms. They are all triploblastic, bilaterally symmetrical, and eucoelomate, In addition, annelids exhibit a body wall with both longitudinal and circular muscle layers (which, along with segmentation mentioned above, allows these animals to be quite mobile), a complete digestive tract, a nervous system showing some degree of cephalization, a closed circulatory system, and an excretory system (based on nephridia).
There are three major classes within the phylum Annelida, described below.
Class Polychaeta - mostly marine worms, such as Nereis (the clamworm)
Class Hirudinea - the leeches (predominantly freshwater), such as Hirudo
Class Oligochaeta - mostly freshwater and terrestrial worms, such as Lumbricus (the earthworms)
In this exercise, we will examine the external and internal structures of Lumbricus. Nereis and Hirudo are available as demonstrations.
Obtain an earthworm from the supply and place the animal in a dissecting tray. You may need 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 a mucous which holds the participants together during sperm exchange.
Orient the worm dorso-ventrally by locating the setae. 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 metamere. 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. Observe the openings for the oviducts on the ventral surface. Find the sperm ducts on the ventral surface of metamere 15.
Pin your worn , 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. Anteriorly there are a number of modifications of the tract. 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 a duct system opening at segment 15. The ovaries are located in segment 13. Their duct system opens to the outside in segment 14. Two pairs of small, round, cream-colored structures are present on the ventro-lateral body wall in segments 9 and 10. These seminal receptacles 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. On reaching segments 9 and 10, sperm from the seminal receptacles are discharged into the sheath and fertilization takes place. The sheath is closed and shed as it reaches the head, forming a cocoon within which direct development (without a larval stage) occurs.
Nephridia are organs of excretion in the annelids. Flood your worm with water. Observe the body wall under a dissecting microscope. The nephridia are coiled tubules with an expanded funnel-shaped nephrostome. The nephrostome is attached to the septum dividing two segments and opens into the anterior segment. The tubule opens to the outside through the body wall within the posterior segment. Filtration of the blood across the tubule wall can occur because of the close association between capillaries and the nephridium.
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. These vessels are the main pumping structures. In the dorsal vessel, blood moves anteriorly. The dorsal vessel is the dark line running along the dorsal surface of the digestive tract. 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.
In the region of your worm where the intestine was removed, locate the ventral nerve cord with its segmental ganglia (seen as slight swellings). Nerve fibers from segmental ganglia innervate structures in each segment. Extend your mid-dorsal incision to the prostomium and locate the paired supra-esophageal ganglia anterior and dorsal to the pharynx. Circum-pharyngeal connectives lead from the supra-esophageal ganglia to the ventral nerve cord.
Using the lowest power of a compound microscope, examine a cross-section of Lumbricus. Note the noncellular, protective cuticle covering 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. 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.
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