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. Annelids show the presence of a true coelom, derived from embryonic mesoderm and protostomy. Hence, they are the most advanced worms. A well-developed and complete digestive system is present in earthworms oligochaetes with a mouth, muscular pharynx, esophagus, crop, and gizzard being present.
The gizzard leads to the intestine and ends in an anal opening. A cross-sectional view of a body segment of an earthworm a terrestrial type of annelid is shown in Figure 2; each segment is limited by a membranous septum that divides the coelomic cavity into a series of compartments. Figure 2. This schematic drawing shows the basic anatomy of annelids in a cross-sectional view. Annelids possess a closed circulatory system of dorsal and ventral blood vessels that run parallel to the alimentary canal as well as capillaries that service individual tissues.
In addition, these vessels are connected by transverse loops in every segment. These animals lack a well-developed respiratory system, and gas exchange occurs across the moist body surface. Annelids show well-developed nervous systems with a nerve ring of fused ganglia present around the pharynx.
The nerve cord is ventral in position and bears enlarged nodes or ganglia in each segment. Annelids may be either monoecious with permanent gonads as in earthworms and leeches or dioecious with temporary or seasonal gonads that develop as in polychaetes. However, cross-fertilization is preferred in hermaphroditic animals. These animals may also show simultaneous hermaphroditism and participate in simultaneous sperm exchange when they are aligned for copulation.
Phylum Annelida contains the class Polychaeta the polychaetes and the class Oligochaeta the earthworms, leeches and their relatives. Unlike flatworms, nematodes are slender, and they are covered by a protective cuticle. A cuticle is a waxy covering secreted by the epidermis, or outermost cellular tissue. Because of this covering, gas exchange cannot occur directly across the skin as in flatworms. Rather, gas exchange and waste excretion in nematodes occurs by diffusion across the wall of the gut.
Although nematodes do have a space in the body between the digestive tract and the body wall, it is not lined with tissue and is not considered to be a true coelom. Thus, nematodes are sometimes referred to as pseudocoelomates Fig. Most worms have two bands of muscles: longitudinal muscles that run the length of the body and circular muscles that form circular bands around the body. Unlike other worms that have two bands of muscles, nematodes only have longitudinal muscles.
This explains their characteristic thrashing movement, as they can move only by contracting the long muscles on either side of their body and wriggling forward. The nervous system of nematodes consists of a set of nerves that run the length of the body and connect to anterior ganglia. Free-living nematodes are capable of sensing light with ocelli, and most nematodes have fairly complex chemosensory abilities.
Most nematodes are not hermaphrodites , with both sexes in one individual, but are known as dioecious —having individuals of separate sexes. Their chemosensory abilities are very helpful, as they rely on pheromones to locate potential mates. The worms in the phylum Annelida from the Latin root word annelus meaning ring typically have complex segmented bodies Fig.
The body of an annelid is divided into repeating sections called segments with many internal organs repeated in each segment. Earthworms class Oligochaeta are familiar terrestrial members of this phylum and leeches class Hirudinea are well-known parasitic members of the phylum, most commonly found in freshwater. They occur mostly in marine and brackish water habitats.
Polychaete from the Greek root words poly meaning many and chaeta meaning bristle annelid worms are so named because most of their segments have bristles called chatae or setae. The free-moving not sessile polychaetes have muscular flaps called parapodia from the Greek para meaning near and podia meaning feet on their sides, and the setae on these parapodia dig into the sand for locomotion. Fireworms are a type of polychaete that have earned their name from stinging bristles on each parapodium Fig.
These bristles can penetrate human skin, causing irritation, pain and swelling, similar to the irritation caused by exposure to fiberglass. Tubeworms are sessile polychaetes that live in tubes that they build by secreting the tube material. The tubes, attached to rocks or embedded in sand or mud, may be leathery, calcareous, or sand-covered depending on the worm species Fig.
Tubeworms feed by extending tentacles from the tube. Bits of food move along grooves in the tentacles to the mouth. Some tubeworms retract their tentacles when food lands on them. Tubeworms use their parapodia to create currents of water that flow through the tubes to aid in respiration and help clean the tubes.
By contrast, the free-living or mobile polychaete worms have a proboscis that can extend from their mouths to catch prey. This is a feeding organ that is often armed with small teeth or jaws on its tip. With their active lifestyle and good defenses, free-moving polychaetes can make their living in a variety of habitats such as mud, sand, sponges, live corals, and algae. Like flatworms, annelids have a mesoderm with muscle, a central nervous system, and an excretory system.
Each of these systems is more complex in the annelid than in flatworms or nematodes. In addition to a more specialized complete digestive system, annelid worms have also evolved body features not found in flatworms or nematodes.
These features appear in some form in all larger, more complex animals:. Recall that the coelom is a fluid-filled cavity lying between the digestive tube and the outer body tube and surrounded by mesodermal tissue. The digestive tube lies inside the outer body tube. The fluid in the coelom supports the soft tissues of the body wall much as it does in the hydrostatic skeleton of cnidarians.
Mesodermal muscles in the wall of the body tube and digestive tube can put pressure on the fluid to aid in movement.
In the body wall of the annelids are two types of muscles: circular and longitudinal. When the circular muscles contract, the segment gets longer and narrower. When the longitudinal muscles contract, the segment gets shorter and fatter Fig. These contractions produce the crawling movement of worms. Recall that nematodes lack circular muscles, and can only move by contracting their longitudinal muscles, thus thrashing and wriggling rather than crawling.
The setae along the body of polychaetes stick in the substrate, holding parts of the worm in place while other parts move forward. Annelids have a closed circulatory system in which blood is pumped along by muscles in blood vessels Fig. Blood flows through the microscopic capillaries, picking up food molecules from the digestive tract and oxygen from the skin and transporting them to the cells of the body.
The parapodia, the flaps on the sides of the segments, increase the surface area of the skin for respiration. Such a system lets animals grow much larger than possible in the flatworms, which must rely on diffusion. The nervous system is also more complex in annelids than in other worm-like phyla.
Annelids have a simple brain organ consisting of a pair of nerve clusters in the head region Fig. Nerves link the brain to sensory organs in the head that detect the environment in front of the worm. Earthworms are eyeless, but polychaete annelids have eyes that can distinguish between light and dark. Some polychaete worm eyes can even detect shapes. Nerves also extend from the brain around the digestive tube and along the ventral surface. They are multicellular organisms that are also eukaryotic; this means that their cells have nuclei.
Annelids are segmented worms. Earthworms belong to this phylum because their bodies are sectioned, creating the ridged or ringed appearance that gives the "ringed worms" of this phylum their name. The name of the class of earthworms owes itself to their clitellum: the collar that serves as a reproductive center during the adult phase of the earthworm's life.
The order of earthworms is either Haplotaxida or Lumbriculida. The distinction depends on whether the earthworm has a freshwater habitat — Lumbriculida — or not — Haplotaxida. There are 16 different families to which a given earthworm may belong. The earthworm genus is generally accepted to be Lumbricus. There are approximately 4, different species of earthworm. You should be able to find out more about a particular earthworm's taxonomy by narrowing it down based on region and habitat.
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