Lumbricus terrestris, the night crawler, is the best known, mostly because it is a favorite fishing bait. Because of its availability and large size, it is a good subject for a laboratory study of oligochaete anatomy.

1. BODY WALL AND COELOM - Outermost is the thin, tough, iridescent cuticle composed of collagen fibers secreted by the epidermis beneath it. The epidermis is a monolayered columnar epithelium containing numerous sensory and secretory cells. A subepithelial nerve plexus lies below the epidermis.

A thick and conspicuous layer of circular muscle lies inside the epidermis. The circular muscle fibers are divided into rings by the circumferential grooves between segments.
Inside the circular muscle layer is a thick layer of white or gray longitudinal muscle. Unlike the circular muscles, the longitudinal fibers run without interruption across segmental boundaries.
The open space enclosed by the two mesothelial layers is the body cavity, or coelom. It is divided into segmental coelomic compartments by double, vertical, transverse sheets of peritoneum, muscle, and connective tissue called septa. Septa are thin and transparent. The septa are penetrated by the gut, hemal system, and nervous system. Longitudinal double sheets of peritoneum extend from the body wall to the gut tube and are mesenteries.

The innermost layer of the body wall is the somatic peritoneum. This is a thin, glistening layer of squamous epithelium that covers the inner surface of the longitudinal muscles. A similar layer of splanchnic peritoneum covers the surface of the visceral organs.

2. DIGESTIVE SYSTEM - The common earthworm's digestive tract is a relatively straight tube with various modifications along its length. Food is taken into the mouth and swallowed by the muscular pharynx behind it; it then passes through the narrow esophagus and enlarged storage-area crop into the muscular gizzard, where, with the help of mineral particles, the food is ground. From the gizzard, the food passes into the intestine, where it is digested and absorbed; the remains are ejected from the anus.

3.CIRCULATORY SYSTEM - The earthworm hemal system contains bright red hemoglobin which makes it very easy to study in living specimens. The pigment is in solution rather than being cellular.
In preview, the basic plan of the system includes two major longitudinal vessels, one dorsal and the other ventral to the. The two vessels are connected in each segment by paired segmental vessels that supply and drain the tissues of the segment. The dorsal vessel and five pairs of segmental vessels in the region of the esophagus are contractile and function as hearts.
Blood moves anteriorly in the dorsal vessel and is pumped to the ventral vessel by the ten segmental hearts. The blood then flows posteriorly in the ventral blood vessel. It leaves the ventral vessel via the segmental vessels of each segment, goes to the organs and then returns to the dorsal blood vessel.
The nervous system is provided with its own system of longitudinal blood vessels supplied by vessels from the ventral blood vessel.
Hemoglobin in invertebrate blood is usually in solution rather than in corpuscles, whereas in the coelom cellular.

4. RESPIRATORY SYSTEM - Earthworms have no specialized respiratory structures and gas exchange takes place across the general epidermis which can be moistened if necessary with fluid from the coelomic pores. The integument is heavily vascularized to serve its respiratory function.

5. NERVOUS SYSTEM - The earthworm nervous system is typically annelidan, consisting of a dorsal, anterior brain, circumpharyngeal connectives, ventral subpharyngeal ganglion and a ventral nerve cord with segmental ganglia, commissures, connectives, and segmental. The brain is farther posterior in oligochaetes than in polychaetes and is no longer in the prostomium.

There are no special sense organs in earthworms but the body surface, especially that of the head, bears receptor cells for taste, touch, light, and apparently vibration. The dorsum of the body is more sensitive to light than the venter. Unicellular photoreceptors are present in the epidermis and are most abundant on the prostomium. These photoreceptors are microscopic and cannot be seen in gross dissection.

6. EXCRETORY SYSTEM - Almost all segments of the earthworm possess a pair of complex metanephridia. They are large, pale, and thin-walled. Each consists of a ciliated funnel, or nephrostome (nephr = kidney, stome = mouth), which opens from the coelomic cavity into a long, tortuously coiled tubule that penetrates the posterior septum of the segment and opens to the outside via a nephridiopore in the wall of the adjacent. The wide distal region of the tubule functions as a bladder.

Biology
A. EXTERNAL ANATOMY

1.CLITELLUM - A band of thickened secretory epithelium, the clitellum (clitell = saddle), girdles the body near the anterior end. The clitellum secretes a mucus cocoon, into which gametes and albumen to nourish the developing embryos are released and where fertilization occurs

2. SEGMENTS - The body is segmented and each segment is separated from its neighbors by a distinct circumferential groove.

The anterior most true segment is the peristomium (peri = around, stome = mouth; Fig 1). Anterior to the peristomium is a small lobe, the prostomium (pro = before) that, for embryological reasons, is not considered to be a segment. The peristomium encircles the large, ventral mouth and is an almost complete ring around the body. The prostomium fits into a small, dorsal notch in the peristomium. The prostomium is a small lobe dorsal to the mouth and anterior to the peristomium. The segments are numbered anterior to posterior beginning with the peristomium.
The segments posterior to the peristomium are complete rings, without notches, and extend uniformly for the entire length of the worm. The posterior most division of the body is the pygidium, which encircles the anus at the posterior tip of the worm. Like the prostomium, the pygidium is not considered a true segment.

3. CHAETAE - The eight small chaetae on each segment are usually visible with adequate magnification (25X). Chaetae are small chitinous bristles emerging from pores in the integument on the ventral half of the worm. The chaetae are arranged in four pairs, two on each side. Each segment has a pair of lateral chaetae and a pair of ventral chaetae on each side. The chaetae are used as anchors when burrowing to hold parts of the worm against so that elongation of the animal results in controlled, usually forward, motion.
The chaetae are retractile and they may be withdrawn in some, or even all, of the segments of the worm. Chaetae are amber or brown, slightly curved, and short.

4. PORES - A little dorsal and anterior to chaeta b on each side of most segments, is a small opening, the nephridiopore, near the anterior edge of the segment. This is the external opening of a metanephridium, or kidney, of which there is a pair in most segments. The nephridiopores are closed by sphincters and are very difficult to see when closed.
Earthworms are simultaneous hermaphrodites and each individual has complete female and male reproductive systems including separate external gonopores.
On the ventral surface of segment 15 are conspicuous swellings associated with the openings of the male vas deferens. The openings themselves are more difficult to see but each is flanked by two low, glandular, transverse ridges, or lips, that are easily seen. The openings are the male gonopores. During copulation sperm from the testes exit the body via these pores. A shallow, inconspicuous sperm groove extends posteriorly from each male gonopore to the clitellum and lies immediately lateral to chaeta b.
The two female gonopores, on segment 14 lateral to chaeta b, are the openings of the oviducts. Eggs from the ovary are released from these pores.
The two pairs of seminal receptacles belonging to the female system are located in segments 9 and 10 and their ducts open laterally in the grooves between segments 9-10 and 10-11 respectively. The four seminal receptacle pores may be difficult to find.

All segments posterior to the twelfth bear a tiny coelomic pore located on the dorsal midline in the groove between adjacent segments. These are almost impossible to see. They are often easier to see along the flatter posterior portion of the worm. They open into the coelom and are used to leak coelomic fluid onto the surface of the animal to keep it moist. Each pore has a sphincter to prevent unnecessary fluid loss.

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