The Nematoda, like other members of the superphylum Ecdysozoa, are triploblastic and possess an embryonic mesoderm that is sandwiched between the ectoderm and endoderm. They are also bilaterally symmetrical, meaning that a longitudinal section will divide them into right and left sides that are superficially symmetrical. In contrast with flatworms, nematodes are pseudocoelomates and show a tubular morphology and circular cross-section.
Nematodes include both free-living and parasitic forms. In , N. Stoll, N. The phylum Nematoda includes more than 28, species with an estimated 16, being parasitic in nature. However, nematologists believe there may be over one million unclassified species.
Nematodes are present in all habitats, typically with each species occurring in great abundance. The free-living nematode, Caenorhabditis elegans , has been extensively used as a model system for many different avenues of biological inquiry in laboratories all over the world.
The cylindrical body form of the nematodes is seen in Figure. These animals have a complete digestive system with a distinct mouth and anus, whereas only one opening is present in the digestive tract of flatworms. The mouth opens into a muscular pharynx and intestine, which leads to a rectum and anal opening at the posterior end. The epidermis can be either a single layer of cells or a syncytium —a multinucleated tissue that in this case is formed by the fusion of many single cells.
The cuticle of nematodes is rich in collagen and a polymer called chitin , which forms a protective armor outside the epidermis. The cuticle extends into both ends of the digestive tract, the pharynx, and rectum. Some nematodes may present other modifications of the cuticle such as rings, head shields, or warts. These external rings, however, do not reflect true internal body segmentation, which as we have seen is a hallmark of phylum Annelida.
The attachment of the muscles of nematodes differs from that of most animals: they have a longitudinal layer only, and their direct attachment to the dorsal and ventral nerve cords creates a strong muscular contraction that results in a whiplike, almost spastic, body movement.
In nematodes, specialized excretory systems are not well developed. Nitrogenous wastes, largely in the form of ammonia , are released directly across the body wall. In some nematodes, osmoregulation and salt balance are performed by simple excretory cells or glands that may be connected to paired canals that release wastes through an anterior pore.
In marine nematodes, the excretory cells are called renette cells , which are unique to nematodes. Most nematodes have four longitudinal nerve cords that run along the length of the body in dorsal, ventral, and lateral positions. The ventral nerve cord is better developed than the dorsal and lateral cords. A similar fusion forms a posterior ganglion at the tail. Nematodes employ a variety of reproductive strategies ranging from monoecious to dioecious to parthenogenetic, depending upon the species.
In the hermaphrodites, ova and sperm develop at different times in the same gonad. Ova are contained in a uterus and amoeboid sperm are contained in a spermatheca "sperm receptacle".
The uterus has an external opening known as the vulva. The female genital pore is near the middle of the body, whereas the male genital pore is nearer to the tip. In anatomical males, specialized structures called copulatory spicules at the tail of the male keep him in place and open the vulva of the female into which the amoeboid sperm travel into the spermatheca. Fertilization is internal, and embryonic development starts very soon after fertilization.
The embryo is released from the vulva during the gastrulation stage. The embryonic development stage lasts for 14 hours; development then continues through four successive larval stages with molting and ecdysis taking place between each stage—L1, L2, L3, and L4—ultimately leading to the development of a young adult worm.
Adverse environmental conditions such as overcrowding or lack of food can result in the formation of an intermediate larval stage known as the dauer larva. Hemocyanin and hemoglobin are the respiratory pigments present in these animals. This subphylum includes animals such as spiders, scorpions, horseshoe crabs, and sea spiders and is predominantly terrestrial, although some marine species also exist. An estimated 77, species, found in almost all habitats, are included in subphylum Chelicerata.
The body of chelicerates may be divided into two parts: prosoma and opisthosoma, which are basically the equivalents of cephalothorax usually smaller and abdomen usually larger. The phylum derives its name from the first pair of appendages, the chelicerae, which are specialized claw-like or fang-like mouthparts. These animals do not possess antennae.
The second pair of appendages is known as pedipalps. In some species, such as sea spiders, an additional pair of appendages, called ovigers, is present between the chelicerae and pedipalps. Chelicerae are used primarily for feeding, but in spiders, these are often modified into fangs that inject venom into their prey before feeding.
Members of this subphylum have an open circulatory system with a heart that pumps blood into the hemocoel. Aquatic species have gills, whereas terrestrial species have either trachea or book lungs for gaseous exchange.
Chelicera of spiders : This photo shows the chelicera of a spider being held open with a stick. Some chelicerae, such as those found in spiders, are hollow and contain or are connected to venom glands which are used to inject venom into prey or a perceived threat. The nervous system in chelicerates consists of a brain and two ventral nerve cords. These animals use external as well as internal fertilization strategies for reproduction, depending upon the species and its habitat. Parental care for the young ranges from absolutely none to relatively-prolonged care.
Privacy Policy. Skip to main content. Search for:. Superphylum Ecdysozoa. Superphylum Ecdysozoa The superphylum Ecdysozoa includes the nematode worms and the arthropods, both of which have a tough external covering called a cuticle.
Learning Objectives Discuss the phylogenetic position of Ecdysozoa. Key Takeaways Key Points The Ecdysozoans are the most diverse group of animals, containing the nematode worms and the arthropods. These organisms have an external covering called a cuticle that protects their soft internal organs from water loss and the outside environment. After they molt, or shed their cuticle, they grow in size and secrete a new shell; this is called ecdysis.
The phylogeny of the Ecdysozoans has been the cause of much scientific debate with no definitive consensus in the scientific community.
Key Terms cuticle : a noncellular protective covering outside the epidermis of many invertebrates and plants coelomate : any animal possessing a fluid-filled cavity within which the digestive system is suspended. Phylum Nematoda Nematodes are parasitic and free-living worms that are able to shed their external cuticle in order to grow.
Learning Objectives Describe the features of animals classified in phylum Nematoda. Key Takeaways Key Points Nematodes are in the same phylogenetic grouping as the arthropods because of the presence of an external cuticle that protects the animal and keeps it from drying out.
There are an estimated 28, species of nematodes, with approximately 16, of them being parasitic. Nematodes are tubular in shape and are considered pseudocoelomates because of they do not possess a true coelom. Nematodes do not have a well-developed excretory system, but do have a complete digestive system. Nematodes possess the ability to shed their exoskeleton in order to grow, a process called ecdysis.
Key Terms exoskeleton : a hard outer structure that provides both structure and protection to creatures such as insects, Crustacea, and Nematoda. Phylum Arthropoda Arthropods are the largest grouping of animals all of which have jointed legs and an exoskeleton made of chitin.
Learning Objectives Describe the morphology of arthropoda. Key Takeaways Key Points Arthropods include the Hexapoda insects , the Crustacea lobsters, crabs, and shrimp , the Chelicerata the spiders and scorpions , and the Myriapoda the centipedes and millipedes. Arthropods have a segmented body plan that contains fused segments divided into regions called tagma.
Arthropods have an open circulatory system and can use book gills, book lungs, or tracheal tubes for respiration. Key Terms tagma : a specialized grouping of arthropodan segments, such as the head, the thorax, and the abdomen with a common function malpighian tubule : a tubule that extends from the alimentary canal to the exterior of the organism, excreting water and wastes in the form of solid nitrogenous compounds spiracle : a pore or opening used especially by spiders and some fish for breathing.
Subphyla of Arthropoda The Phylum Arthropoda includes a wide range of species divided into the subphyla: Hexapoda, Crustacea, Myriapoda, and Chelicerata.
Learning Objectives Differentiate among the subphylums hexapoda, myriapoda, crustacea, and chelicerata. They take their name from their round body cross-sectional shape.
Unlike the flatworms in which food and waste enter and exit from the same opening, nematodes have a complete digestive system. An animal with a complete digestive system has a mouth at one end, a long tube with specialized parts in the middle, and an anus at the other end. With a complete digestive system an animal can eat while its previous meal digests. Parts of the digestive system can specialize to do different jobs, digesting food in stages Fig.
As the food moves along, it is broken into molecules and absorbed by the cells lining the tube. Muscles surrounding the tube contract, squeezing the food and pushing it along in a process called peristalsis. Indigestible wastes pass out through the anus. 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. The worms may produce enzymes that digest the host tissues or graze on surface mucus and skin particles.
Most monogeneans are hermaphroditic, but the sperm develop first, and it is typical for them to mate between individuals and not to self-fertilize. The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans.
Trematodes have complex life cycles that involve a primary host in which sexual reproduction occurs and one or more secondary hosts in which asexual reproduction occurs. The primary host is almost always a mollusk.
Trematodes are responsible for serious human diseases including schistosomiasis, caused by a blood fluke Schistosoma. The disease infects an estimated million people in the tropics and leads to organ damage and chronic symptoms including fatigue. Infection occurs when a human enters the water, and a larva, released from the primary snail host, locates and penetrates the skin. The parasite infects various organs in the body and feeds on red blood cells before reproducing. Many of the eggs are released in feces and find their way into a waterway where they are able to reinfect the primary snail host.
The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates. Tapeworms live in the intestinal tract of the primary host and remain fixed using a sucker on the anterior end, or scolex, of the tapeworm body.
The remaining body of the tapeworm is made up of a long series of units called proglottids, each of which may contain an excretory system with flame cells, but will contain reproductive structures, both male and female.
Most reproduction occurs by cross-fertilization. The proglottid detaches and is released in the feces of the host. The fertilized eggs are eaten by an intermediate host. The juvenile worms emerge and infect the intermediate host, taking up residence, usually in muscle tissue. When the muscle tissue is eaten by the primary host, the cycle is completed.
There are several tapeworm parasites of humans that are acquired by eating uncooked or poorly cooked pork, beef, and fish. The phylum Nematoda , or roundworms, includes more than 28, species with an estimated 16, parasitic species. Most nematodes look similar to each other: slender tubes, tapered at each end [Figure 3]. Nematodes are pseudocoelomates and have a complete digestive system with a distinct mouth and anus.
The nematode body is encased in a cuticle, a flexible but tough exoskeleton, or external skeleton, which offers protection and support. The cuticle contains a carbohydrate-protein polymer called chitin. The cuticle also lines the pharynx and rectum. Although the exoskeleton provides protection, it restricts growth, and therefore must be continually shed and replaced as the animal increases in size.
There may also be a sharp stylet that can protrude from the mouth to stab prey or pierce plant or animal cells. The mouth leads to a muscular pharynx and intestine, leading to the rectum and anal opening at the posterior end. In nematodes, the excretory system is not specialized.
Nitrogenous wastes are removed by diffusion. In marine nematodes, regulation of water and salt is achieved by specialized glands that remove unwanted ions while maintaining internal body fluid concentrations. Most nematodes have four nerve cords that run along the length of the body on the top, bottom, and sides. Beneath the epidermis lies a layer of longitudinal muscles that permits only side-to-side, wave-like undulation of the body. View this video to see nematodes move about and feed on bacteria.
Nematodes employ a diversity of sexual reproductive strategies depending on the species; they may be monoecious, dioecious separate sexes , or may reproduce asexually by parthenogenesis.
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