A ventral nerve cord extends the length of the animal and connects to a pair of fused ganglia mass of nervous tissue in each segment. The ganglia within each segment function to coordinate muscle contractions. Examine the exterior of an earthworm and find the ventral surface. Place the worm in a dissecting pan with the ventral surface down. Identify the clitellum. This structure produces mucous needed for reproduction.
Find rows of setae along either side of the ventral surface. These help provide traction as the animal moves through the soil. Find the dorsal blood vessel. This structure should be visible through the body wall and will appear slightly darker than the rest of the body. It extends the length of the animal. With a scalpel, make a shallow cut along the dorsal surface beginning at a point approximately half way between the clitellum see photograph below and the posterior end and ending at the anterior end.
Try to avoid cutting the dorsal blood vessel by keeping your cut to one side of the vessel. Try to cut only the body wall but not the internal structures.
The septa between each segment will prevent you from spreading the walls open to view the interior structures. Use a needle or scalpel to carefully cut the septa while spreading the body open.
Pins can be used to hold the body open. Find the structures shown in the photographs below. Earthworms are simultaneous hermaphrodites and each individual has complete female and male reproductive systems including separate external gonopores.
The genital pores are on the venter of segments 14 female and 15 male. The male genital pores are flanked by two low ridges, so are more easily seen. Segments posterior to the twelfth have a tiny coelomic pore on the dorsal midline in the groove between adjacent segments. These are very difficult to find. They 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.
While holding the preserved worm, use your finest scissors to make a shallow incision a little to one side of the dorsal midline. Start by pinching the body wall with forceps and then cut through the pinch with the scissors. Extend the dorsal incision anteriorly to the prostomium and posteriorly to about segment , keeping the incision a little to the side of the dorsal midline.
Position the worm in a wax pan and pin the worm open with pins angled at about 45degrees to keep the pins out of the way as much as possible. The body cavity , or coelom , is partitioned by septa , which are transverse sheets of thin tissue that extend from the body wall to the gut tube.
A thick and conspicuous layer of circular muscle lies inside the epidermis. Inside the circular muscle layer is a thick layer of white or gray longitudinal muscle. The digestive system is a straight tube extending from mouth to anus. The gut is regionally specialized to perform the several functions, such as food procurement, storage, grinding, digestion, absorption, and feces formation. The mouth opens into the short, thin-walled buccal cavity in segments The pharynx is posterior to the buccal cavity in segments The pharyngeal wall is thick and muscular.
Numerous small radial muscles run from the pharynx to the body wall. Contraction of these muscles dilates the pharynx. The conspicuous, white cerebral ganglia brain is located atop the junction between the buccal cavity and the pharynx.
Posterior to the pharynx, the gut narrows to become the thin-walled esophagus in segments The posterior end of the esophagus is hidden by six large, creamy white seminal vesicles that arch over it. The esophagus narrows and joins the large, bulbous, thin-walled crop in about segment The crop is a food storage organ.
Posterior to the crop is the gizzard. The gizzard has thick, heavy, muscular walls for grinding food into smaller fragments. The buccal cavity, pharynx, esophagus, crop, and gizzard form the foregut. Posterior to the gizzard, the gut narrows again and becomes the intestine or midgut , which is the region for chemical digestion and absorption. The intestine extends from the end of the gizzard almost to the anus and its dorsal wall is invaginated to increase surface area.
The extreme posterior end of the gut is the rectum , or hindgut. It opens to the exterior via the anus. The basic plan of the circulatory system includes dorsal and ventral longitudinal vessels, which are connected in each segment by paired segmental vessels. The dorsal vessel and five pairs of segmental vessels in the region of the esophagus are contractile and function as hearts.
Blood flows anteriorly in the dorsal vessel where one-way valves prevent backflow and posteriorly in the ventral vessel. Find the large dorsal blood vessel on the midline of the dorsal surface of the gut.
Follow the dorsal vessel anteriorly to the esophagus and find the five pairs of large, contractile, segmental blood vessels which function as hearts. They pump blood ventrally, which is the reverse of the other segmental vessels. The hearts may be hidden by the septa and nephridia of their segments.
Remove these as necessary to reveal the hearts. The ventral blood vessel is attached to the ventral side of the gut by the narrow mesentery. 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. The earthworm nervous system is consists of a dorsal, anterior brain, circumpharyngeal connectives, ventral subpharyngeal ganglion and a ventral nerve cord with segmental ganglia, commissures, connectives, and segmental nerves.
The brain , consists of a pair of cerebral ganglia above the anterior pharynx in segment 3. If your previous incision exposed the buccal cavity, the brain is visible without further dissection.
Each ganglion is white and pear-shaped, and the two are connected across the midline. The brain is the primary center for coordination of sensory and motor functions. A large circumpharyngeal connective exits the side of each cerebral ganglion and runs ventrally around the pharynx to join the subpharyngeal ganglion ventral to the pharynx.
We will not dissect out this ganglion, which is a major center for motor control. The double, solid ventral nerve cord extends posteriorly, on the ventral midline, for the length of the worm.
The nerve cord swells in each segment to form a segmental ganglion. 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 vibration. Almost all segments of the earthworm possess a pair of complex metanephridia on the sides of each segment. These may be difficult to see in preserved specimens. Oligochaetes are simultaneous hermaphrodites and each individual contains complete and simultaneously functional male and female systems.
The female system produces eggs and receives and stores sperm from the partner. The male system produces sperm and delivers it to a partner during copulation.
The reproductive system is restricted to a few preclitellar segments These animals are found in marine, terrestrial, and freshwater habitats, but a presence of water or humidity is a critical factor for their survival, especially in terrestrial habitats. The name of the phylum is derived from the Latin word annellus , which means a small ring.
Animals in this phylum show parasitic and commensal symbioses with other species in their habitat. Approximately 16, species have been described in phylum Annelida. The phylum includes earthworms, polychaete worms, and leeches. Figure 1. The clitellum, seen here as a protruding segment with different coloration than the rest of the body, is a structure that aids in annelid reproduction.
Annelids display bilateral symmetry and are worm-like in overall morphology. Annelids have a segmented body plan wherein the internal and external morphological features are repeated in each body segment. This metamerism is thought to arise from identical teloblast cells in the embryonic stage, which give rise to identical mesodermal structures. The overall body can be divided into head, body, and pygidium or tail. The clitellum is a reproductive structure that generates mucus that aids in sperm transfer and gives rise to a cocoon within which fertilization occurs; it appears as a fused band in the anterior third of the animal Figure 1.
The epidermis is protected by an acellular, external cuticle, but this is much thinner than the cuticle found in the ecdysozoans and does not require periodic shedding for growth.
Circular as well as longitudinal muscles are located interior to the epidermis. Source : Hickman, C.
Animal Diversity. Brown, Dubuque, IA. Read more Classification Kingdom Animalia animals Animalia: information 1 Animalia: pictures Animalia: specimens Animalia: sounds Animalia: maps Annelida: information 1 Annelida: pictures Related Taxa Species Hirudo medicinalis Hirudo medicinalis: information 1 Hirudo medicinalis: pictures 1. Lamellibrachia luymesi: information 1 Lamellibrachia luymesi: pictures 1.
Polychaeta: information 1 Polychaeta: pictures Riftia pachyptila: information 1 Riftia pachyptila: pictures 1.
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