Workshop in Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Annelids
Class Oligochaeta, or earthworms

Earthworm (Lumbricus terrestris).

Earthworms inhabit moist, humus-rich soil. They feed on soil organic matter by passing earth containing plant debris through their digestive tract. The Role of earthworms in enriching the soil with organic matter, modifying its Structure, and ensuring proper aeration is substantial.

Materials and Equipment. The fauna of our country features numerous species of earthworms that are easily collected, survive well in captivity, and are sufficiently large for dissection.

Worms are obtained by digging them out of the soil or by collecting them beneath stones, rotting logs, or pieces of turf lying on the ground. Upon spotting an earthworm, one must swiftly press a hand onto the ground behind the posterior end of its body as it retreats into its burrow. The remaining portion of the body should then be excavated using a trowel or spade. This collection method succeeds only with prompt action, as earthworms are sensitive to light, soil vibrations, and other stimuli. In captivity, earthworms are kept in a wooden box filled with soil. The box is filled to three-quarters capacity with alternating layers of garden soil and previous year's leaves, supplemented with a small amount of horse or rabbit manure. Provide 3 to 5 buckets of soil per hundred worms. The soil is then lightly moistened with Water, and the worms are introduced into the box. To retain moisture, it is helpful to cover the soil with several layers of burlap.

The covered box should be stored in a semi-dark Location at a Temperature of 12–15°C. As the soil dries out, it should be lightly moistened. To feed the worms, periodically add chopped decaying leaves, potato peelings, or mashed boiled potatoes.

Prior to examining external Morphology or performing dissection, the worms are euthanized in chloroform vapor or by immersion in 15–20% alcohol.

Required apparatus and supplies: a Microscope; a dissecting loupe; a hand lens; dissection tools (scalpel, razor blade, scissors, forceps, dissecting tray, needles, pins); Glass slides and coverslips; a beaker or jar of water; 1/2 sheet of white paper; live and ether- or 15–20% alcohol-euthanized large earthworms; a wet mount of a dissected earthworm; a microscopic slide of a worm cross-section.

Assignment. Observe the movements of an earthworm. Examine its external morphology. Dissect an earthworm, studying and sketching its internal anatomy. Study and sketch a cross-section of an earthworm.

Observation of Living Specimens and Study of Fixed Material. Place a live earthworm on a sheet of paper. Determine the ventral and dorsal sides, as well as the anterior and posterior ends of the body. Observe the worm's locomotion. Pay attention to the pattern of muscular contraction and the rustling sound produced by the movement of the setae against the paper.

Examine an earthworm (alive or freshly euthanized with ether or alcohol) using a hand lens or a binocular dissecting microscope. The body of the worm reaches 10–30 cm in length.

Note that it consists of numerous rings, or segments (annuli), separated from one another by constrictions (Fig. 61). The dorsal side is rounded and darker, with the dorsal Blood vessel faintly visible through the Skin; the ventral side is flattened and lighter in color. Examine the anterior end of the body; it is thicker than the posterior end and more darkly pigmented. Across segments 32–37, the body thickens to form the clitellum, which is rich in mucous glands. The clitellum plays a vital role in reproduction.

Locate the Mouth, situated on the ventral side in the 2nd segment. To observe the oral opening, use the following technique: hold the fixed worm with the thumb and forefinger of your left hand near the anterior end so that the 1st and 2nd body segments project above your fingers. Squeeze your fingers gently. Through the loupe, examine the mouth opening positioned close to the ventral side and the prostomium—a lobe-like PROJECTION OF THE upper part of the 1st segment covering the mouth.

Locate the anus at the posterior end of the body; it appears as a small slit. Note that each segment bears two pairs of resilient setae, forming four longitudinal rows of paired setae along the body. The setae function as levers to facilitate locomotion and help anchor the worm within its burrows. Formed by the cuticle, the setae consist of a Chitin-like chemical composition. Setae are easily detected by running a finger along the ventral surface of the worm from the posterior to the anterior end. This tactile inspection also reveals that the worm's body is covered by a smooth, glossy cuticle and is rich in mucous glands. Abundant mucus on the body surface protects the worm from desiccation and facilitates crawling through burrows by reducing friction against the soil. Thanks to the mucus, the worm's skin remains constantly moist—a critical factor for Respiration, as gas exchange between the Organism and the external environment occurs across the entire body surface. The skin is densely supplied with sensory nerve endings, allowing the earthworm to readily detect mechanical, light, and chemical stimuli despite lacking anatomically distinct Sense Organs. Sketch the External structure of the earthworm from the ventral view, labeling the setae, body segments, mouth, clitellum, and anus.

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Fig. 61 External appearance of the anterior end of an earthworm:

1 — prostomium; 2 — lateral setae; 3 — ventral setae; 4 — male genital pore; 5 — female genital pore; 6 — seminal groove; 7 — clitellum.

Dissection. Study the Organ Systems concurrently using the dissected worm and the wet mount. Place the earthworm in the dissecting tray with its dorsal side up, stretch it slightly, and pin down the anterior and posterior ends of the body. Anchor the HEAD end with two pins inserted at the level of the 3rd segment to avoid damaging the cerebral ganglion. Make a longitudinal incision in the body wall (dermomuscular tube) using a scalpel or razor blade along the dorsal midline for a length of 1–2 cm (cutting parallel to the dorsal blood vessel without injuring it). Extend the incision toward the anterior and posterior ends using scissors, taking care not to damage the gut (extreme caution is required when opening the anterior region). Hold the edges of the incision apart with forceps while trimming the transverse septa (dissepiments) with a scalpel. Then, reflect the body walls outward and pin them to the bottom of the tray. Pins should be inserted in pairs without overstretching the skin to prevent tearing. Incising the body wall provides access to the secondary body cavity, or coelom, where the alimentary canal is clearly visible (Fig. 62).

Fig. 62 Anatomy of an earthworm:

1 — prostomium (oral lobe); 2 — supra-oesophageal ganglion; 3 — Pharynx;

4 — oesophagus; 5, 13 — lateral hearts (ring vessels); 6 — dorsal vessel; 7 — Seminal Vesicles; 8 — Testes;

9 — sperm funnels; 10 — vas deferens; 11 — intersegmental septa; 12 — metaneridia; 14 — intestine; 15 — gizzard; 16 — crop; 17 — oviduct; 18 — egg funnels; 19 — Ovary; 20 — spermathecae.

Examine the delicate transverse partitions, or dissepiments, along the sides of the intestine that divide the body cavity into individual compartments corresponding to the body segments.

Pour water into the dissecting tray until the worm is submerged. Begin studying the Internal Organs with the aid of a loupe. Examine the Digestive System (Fig. 63).

The mouth leads into a muscular pharynx (located in segments 2–6). Trace how the pharynx transitions into a relatively long oesophagus (segments 7–13), into which three pairs of calciferous glands—Morren's glands—open in segments 10, 11, and 12 (though not always distinctly defined, they can sometimes be seen by lifting the seminal vesicles). As noted above, earthworms feed on soil organic matter by ingesting earth rich in humus and humic acids. Morren's glands secrete lime, which is released into the oesophagus to neutralize the dietary humic acids.

Fig. 63

Cytology/practical/54.html">Longitudinal section of the anterior part of an earthworm's body:

1 — septa; 2 — seminal vesicles; 3 — metanephridia; 4 — Esophagus; 5 — pharynx; 6 — mouth; 7 — supraesophageal ganglion; 8 — ventral nerve cord; 9 — crop; 10 — gizzard; 11 — midgut.

Locate the crop (an expansion of the esophagus) in the region of the 14th–15th segments, followed by the thick-walled muscular gizzard, which serves for the mechanical Digestion of food. Trace how the muscular gizzard transitions into the very long midgut. By making a transverse cut across the midgut somewhere in the posterior part of the body, you can see that it features a longitudinal fold on its dorsal side—the typhlosole—which increases the surface area available for digestion. The midgut passes imperceptibly into the short hindgut, which terminates in the anus.

Examine the main Blood Vessels, which stand out clearly against the Background of the gut as bright red in a living or freshly killed worm. The earthworm's blood is red due to Hemoglobin dissolved in the plasma, which is closely related to the hemoglobin found in the blood of vertebrates. Find the dorsal blood vessel running along the DORSAL SIDE OF the gut, through which blood flows from the posterior end of the body toward the anterior. Lift the severed portion of the gut to reveal the longitudinal ventral blood vessel running beneath it, in which blood moves from the anterior end of the body toward the posterior. You can see that both vessels are connected by numerous ring vessels that loop around the walls of the gut. Particularly robust vessels are located in the esophageal region—these are the so-called "hearts" (see Fig. 62). The rhythmic pulsation of their muscular walls drives the Circulation of blood through the vessels. The arrangement of some smaller longitudinal vessels is best studied in a transverse section of the worm. The earthworm has a closed Circulatory system.

Respiratory organs are absent. Respiration occurs through the skin, which is permeated by a dense network of blood capillaries. Oxygen diffuses through the moist skin surface into the blood flowing through the capillaries, while carbon dioxide—accumulated in the Tissues As a result of cellular respiration—passes from the blood into the external environment.

Using a magnifying glass and gently rocking the dissecting tray with the opened worm, examine the thin, whitish, convoluted tubules located between the septa on both sides of the gut. These are the excretory organs—the metanephridia (Fig. 64). Each segment (except the first three and the last one) contains a pair of metanephridia, which is why they are sometimes called segmental organs. To examine The structure of a metanephridium, cut out a septum along with its attached segmental organ, place it on a glass slide in a drop of water, and observe it under low magnification using a microscope.

A metanephridium begins with a small funnel—the nephrostome—the edges of which bear numerous cilia. The wide end of the funnel opens into the body cavity, while the narrow end pierces the septum and connects (in the next segment) to the excretory tubule, which forms several loops, expands into a vesicle, and opens to the outside through a specialized pore.

Fig. 64 Structure of metanephridia in Annelids:

1 — intersegmental partitions (septa); 2 — ciliated nephridial funnels (nephrostomes);

3 — excretory tubule; 4 — opening of the excretory tubule.

The funnels themselves are not visible; only the excretory canal can be observed.

The metanephridia are arranged in pairs: one on the left side of the gut, the other on the right. Thus, in each segment of the earthworm (except the first three and the final posterior one), There is a pair of excretory pores on the ventral side (though they are difficult to distinguish). Final Metabolic waste products pass from the body Cells and Tissues into the coelomic fluid and are then removed to the exterior by the metanephridia. In addition to the segmental organs, chloragogen cells—which cover The surface of the gut with a thin brownish-yellow coating—participate in excretion. If you gently scrape the surface of the gut with a dissecting needle, a fine turbidity consisting of these cells detaches. Chloragogen cells accumulate metabolic waste products (excretes). Once filled with excretes, these cells die, and their contents enter the body cavity to be eliminated by the metanephridia.

The Reproductive System of the earthworm is located in the region of the esophagus and crop (segments 9–15). Earthworms are hermaphrodites. Not all PARTS OF THE reproductive apparatus can be seen with the unaided eye. The Gonads (testes and Ovaries) can generally be observed only during the breeding season (at other times they are so small that they are difficult to distinguish). Most striking are the three pairs of large, whitish seminal vesicles situated in segments 10, 11, and 12. Their medial ends fuse into a single unpaired pouch, which is covered dorsally by the esophagus.

The seminal vesicles serve as a reservoir for spermatozoa produced by two pairs of testes located inside the unpaired portion of the seminal vesicles. Opposite each Testis lies a small funnel connected to a sperm duct. Both ducts on the right side fuse into the right vas deferens, and those on the left side into the left vas deferens. The vasa deferentia open to the exterior on the ventral side of the 15th segment via a pair of male genital pores. The sperm ducts cannot be seen; they are indistinguishable in a standard slide preparation.

The FEMALE REPRODUCTIVE SYSTEM consists of a pair of ovaries located in the 13th segment. Nearby are the funnels of the oviducts, which open to the exterior in the 14th segment through a pair of female genital pores. The oviducts cannot be found with the unaided eye, even during the breeding season. The female reproductive system also includes two pairs of spermathecae (in segments 9 and 10); each opens to the exterior through a pore.

Examine the spermathecae with a magnifying glass after gently pushing aside the seminal vesicles with forceps. Fertilization takes place as follows. Two worms encountering each other in their underground burrows apply their ventral surfaces together so that their posterior ends point in opposite directions, and they are glued together by mucus secreted by the clitellum. Each partner releases droplets of sperm, which are sucked into the spermathecae of the other partner. After exchanging sperm, the worms separate.

By the time of egg-laying, the worm's clitellum secretes mucus that forms a band-like tube (cocoon). As the earthworm crawls backward out of this mucous sleeve, it deposits eggs into it along with a drop of sperm from its former mating partner. Once the worm slips out of the mucous tube, its edges seal shut, transforming it into a cocoon within which The Development of the young worms takes place.

The Nervous system of the earthworm consists of supraesophageal and subesophageal ganglia connected by commissures, as well as a ventral nerve cord. The supraesophageal, or cerebral, ganglion consists of two fused nodes. The cerebral and subesophageal ganglia, along with their commissures, form the circumpharyngeal nerve ring. Remove a portion of the gut and examine the ventral nerve cord using a magnifying glass. The ventral nerve cord consists of segmentally arranged ganglia linked to one another by commissures. In a dissected worm, it has a yellowish-white color.

Make a drawing of the internal Anatomy of the earthworm.

Study of a transverse section slide (see Fig. 65).

Examine the prepared slide of a body section under a magnifying glass and low microscope magnification, and make a drawing of it. Orient the slide by identifying the ventral and dorsal sides. This is easily done by remembering that the intestinal fold—the typhlosole—is located on the dorsal side of the gut.

Examine the STRUCTURE OF THE body wall: the exterior of the body is covered by a thin cuticle, beneath which lies a layer of epithelial cells, followed by the musculature. The musculature consists of an outer circular layer and an inner longitudinal layer. Together, these form the musculocutaneous sac. The inner surface of the musculocutaneous sac is lined with epithelium. The presence of this epithelial lining clearly distinguishes the coelom (secondary body cavity) from a primary body cavity.

Locate and examine on a cross-section, using Figure 65, the dorsal blood vessel, the intestine covered externally with chloragogen cells, the typhlosole, the typhlosolar blood vessel, the ventral blood vessel, the ventral nerve cord, as well as the subneural vessel and metanephridia (various parts of the segmental organs may be visible in the section, most frequently convoluted tubules).

Fig. 65 Cross-section of the body of an earthworm:

1 — metanephridium; 2 — nephrostome; 3 — ganglion of the ventral nerve cord; 4 — epidermal epithelium; 5 — circular Muscles; 6 — longitudinal muscles; 7 — setae; 8 — intestinal dorsal fold; 9, 10 — dorsal and ventral blood vessels; 11 — subneural vessel.