How Many of the Following Can Be Used to Distinguish a Nematode Worm From an Annelid Worm?
Introduction to Worms
Most people are familiar with earthworms found in garden soil. Although many different kinds of animals are usually lumped together as "worms," there are several distinct phyla that fit the category. Worms are typically long, thin creatures that go around efficiently without legs. The different phyla of worms display a swell range in size, complexity, and body construction. Flatworms (phylum Platyhelminthes) are simple animals that are slightly more than circuitous than a cnidarian. Roundworms (phylum Nematoda) have a slightly more complex body program. Segmented worms (phylum Annelida) are the most complex animals with worm-like body plans. A study of worms can illuminate a possible history of how some organ systems and body features evolved.
Worms are invertebrate animals with bilateral symmetry. Worms have a definite anterior (head) cease and a posterior (tail) end. The ventral surface of worms and other organisms is the bottom side of the trunk, often closest to the ground. The dorsal surface is located on the upper office of the trunk facing the sky. The lateral surfaces are found on the left and right sides of the body. Figure 3.35 compares bilateral symmetry in a whale shark and a swimming plychaete worm. Organs for sensing light, touch, and smell are concentrated in the heads of worms. They tin detect the kinds of environment they encounter by moving in the anterior management.
There are six features and systems that reveal an evolving complication in the body structure of near worms:
- a mesoderm, an intermediate body layer between the inner (endoderm) and outer (ectoderm) tissue layers that forms musculus tissue
- a central nervous organisation guided by a "encephalon"
- an excretory system to eliminate some kinds of waste products
- a consummate digestive system, from an anterior mouth to a posterior anus
- a coelom, a body cavity between the digestive tube and the external trunk wall that is lined with tissue
- a circulatory system consisting of a series of tubes (vessels) filled with fluid (blood) to ship dissolved nutrients, oxygen, and waste material products effectually the body quickly and efficiently
Flatworms: Phylum Platyhelminthes
The phylum Platyhelminthes consists of simple worm-similar animals called flatworms (Fig. 3.36). The name Platyhelminthes (pronounced "plat-ee-hel-MIN-theze") is derived from the Greek root word platy meaning flat and the Greek root give-and-take helminth meaning worm. Flatworms alive on land, in fresh water, in the ocean, and in or on other animals as parasites (e.g., tapeworms). Parasitic flatworms that live on or within other animals—including humans—can hurt or even kill the host organism. Gratis-living non-parasitic flatworms are typically less than 10 centimeters long. Marine species alive cached in the sand or under rocks in shallow water. All free-living flatworms are predators that actively chase for food. Some live symbiotically with crabs, clams, oysters, shrimp, and barnacles. Some marine flatworms are brilliantly colored (Fig. 3.36 A) while others are drab and blend into the environment (Fig. iii.36 B).
Flatworms are more complex than cnidarians. Cnidarians accept two layers of cells, the ectoderm and the endoderm; flatworms have a middle layer called the mesoderm between the other 2 layers (Fig. 3.xvi). This extra layer is important because its cells specialize into a muscular system that enables an animal to move around. Beginning with the flatworms, all the animals we will afterwards written report have a mesoderm and muscular system. The cells of the ectoderm and endoderm are as well more than organized than similar cells of cnidarians. For the kickoff time, we run across groups of tissues that have evolved to class organs, such as the ones in the digestive, nervous, and excretory systems.
Like the cnidarians, flatworms have a digestive organization with only a single opening into the digestive cavity, merely in independently living marine flatworms the cavity branches into all parts of the trunk (Fig. 3.37 B). These flatworms feed through a throat. A pharynx is a long, tubular mouthpart that extends from the body, surrounds the nutrient, and tears it into very fine pieces (Fig. 3.37 C and D). Cells lining the digestive cavity finish digesting the food. Then the dissolved nutrients motility to other cells of the trunk. Undigested food passes back out through the mouth, as in the cnidarians. Parasitic tapeworms normally absorb their nutrients directly from the host, while parasitic flukes have retained a digestive organisation.
Like most self-propelling animals, independent-living flatworms accept a cardinal nervous organisation. A central nervous organisation consists of a mass of nerve cells, called a ganglion, (in more complex organisms, the ganglion evolves into a encephalon) in the inductive role of the body, and a nerve cord extending from the brain toward the posterior cease of the trunk (Fig. iii.38). Sensory cells in the head notice changes in the environment. In free-living flatworms, sensory cells that respond to light are amassed in 2 eyespots in the head. Sensory cells that detect water currents, solid objects, and chemicals are in ii flap-similar projections on the caput chosen auricles. In self-propelling animals, these sensory organs in the head are the get-go part of the animal that encounters new environs. The ganglion receives information from the sensory structures and sends signals to other parts of the trunk forth two strands of nerve cells running toward the tail. Because the nerve strands are connected by cross-strands in the shape of a stepladder, this kind of nervous system is ofttimes called a "nerve ladder."
The excretory organization removes waste products and backlog water from tissues of flatworms. Flatworms accept a surprisingly elaborate organisation to rid the body of wastes (Fig. three.39). This network runs the length of the animate being on each side and opens to the exterior through small pores in the posterior region of the body. Continued to the tubes are tiny cells that movement wastes and h2o from the tissues into the tubes. These cells contain flagella that beat back and forth, creating a current of fluid that constantly moves toward the excretory pores. Nether a microscope the flagellar movement looks similar a flickering fire, and the structure is called a flame bulb.
Flatworms take no circulatory system. Animals without a circulatory system have express abilities to deliver oxygen and nutrients to their body cells because of the way that molecules behave. As molecules spread through water, they become less concentrated equally they move away from their source. This is known as improvidence. A brawl-shaped marine animal would not get acceptable oxygen and nutrients to its innermost cells considering the cells are too far from the torso's surface for molecules to move (diffuse) to them (Fig. 3.40 A). Just cnidarians take no problem with diffusion considering most cells of their handbag-shaped bodies are in direct contact with the water, making the commutation of oxygen and nutrients like shooting fish in a barrel (Fig. 3.xl B). Flatworms, bag-shaped just flattened, as well get oxygen and nutrients to their torso cells hands because all their cells are close to either their outer surface or their digestive crenel (Fig. three.40 C). Equally animals become larger and more complex, diffusion is often no longer an option, so we brainstorm to see the evolution of circulatory and respiratory systems.
Roundworms: Phylum Nematoda
Species in the phylum Nematoda (from the Greek root give-and-take nema meaning thread) are better known as the roundworms (Fig. iii.41). There are about 25,000 species of nematodes formally described past scientists. Nematodes are institute in almost every habitat on Globe. One species was starting time discovered living inside felt beer coasters in German alehouses. Studies of farmlands accept found equally many as ten,000 nematodes in 100 cubic centimeters (cm3) of soil. Nematodes are similarly arable in marine and freshwater sediments where they serve as important predators, decomposers, and prey for other species similar crabs and snails.
Like flatworms, roundworm species adopt either a gratis-living or a parasitic lifestyle. Parasitic nematodes (Fig. 3.41 A, C, D, and E) include heartworms that infect domestic dogs and the hookworms and pinworms that commonly infect small children. Many nematodes that are parasitic on plants can devastate crops. Some nematodes are cryptobiotic and take demonstrated a remarkable ability to remain dormant for decades until environmental conditions become favorable.
Like the flatworms, nematodes are bilaterally symmetrical. They have their name from their circular body cantankerous-sectional shape. Dissimilar the flatworms in which food and waste product enter and go out from the same opening, nematodes have a complete digestive system. An animal with a complete digestive arrangement has a mouth at 1 cease, a long tube with specialized parts in the middle, and an anus at the other terminate. Complete digestive systems are seen in more complex organisms and offer many advantages over the flatworm'southward method of digestion. With a complete digestive system an creature can eat while its previous meal digests. Parts of the digestive system can specialize to do dissimilar jobs, digesting nutrient in stages (Fig. three.42). As the nutrient moves along, it is broken into molecules and absorbed past the cells lining the tube. Muscles surrounding the tube contract, squeezing the nutrient and pushing information technology 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 beyond the peel as in flatworms. Rather, gas commutation and waste matter excretion in nematodes occurs past diffusion across the wall of the gut. Although nematodes practice accept a space in the trunk between the digestive tract and the torso wall, it is not lined with tissue and is not considered to exist a true coelom. Thus, nematodes are sometimes referred to as pseudocoelomates (Fig. 3.17 C).
Most worms accept 2 bands of muscles: longitudinal muscles that run the length of the body and circular muscles that form round bands around the body. Unlike other worms that have two bands of muscles, nematodes only have longitudinal muscles. This explains their characteristic thrashing movement, equally they tin 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 trunk and connect to anterior ganglia. Gratis-living nematodes are capable of sensing light with ocelli, and about nematodes have fairly complex chemosensory abilities. Nigh 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.
Segmented Worms: Phylum Annelida
The worms in the phylum Annelida (from the Latin root word annelus meaning ring) typically have complex segmented bodies (Fig. 3.43). 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 (form Hirudinea) are well-known parasitic members of the phylum, most commonly found in freshwater. The polychaete worms or "bristleworms" (course Polychaeta) are the largest group in the phylum Annelida. They occur mostly in marine and stagnant water habitats.
Image courtesy of Hans Hillewaert, Wikimedia Commons
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Polychaete (from the Greek root words poly significant many and chaeta meaning bristle) annelid worms are so named considering most of their segments have bristles called chatae or setae. Effigy 3.44 shows two examples of polychaete setae. The free-moving (not sessile) polychaetes have muscular flaps chosen 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 take earned their name from stinging bristles on each parapodium (Fig. 3.44 A). These bristles can penetrate homo skin, causing irritation, pain and swelling, similar to the irritation caused by exposure to fiberglass.
Tubeworms are sessile polychaetes that alive 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. 3.45). Tubeworms feed by extending tentacles from the tube. Bits of food motility along grooves in the tentacles to the mouth. Some tubeworms retract their tentacles when food lands on them. Tubeworms employ their parapodia to create currents of water that catamenia through the tubes to help in respiration and assistance clean the tubes. By dissimilarity, 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 oft armed with small teeth or jaws on its tip. With their active lifestyle and good defenses, costless-moving polychaetes tin make their living in a diverseness of habitats such as mud, sand, sponges, live corals, and algae.
Like flatworms, annelids have a mesoderm with musculus, 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 than specialized complete digestive system, annelid worms have likewise evolved torso features non found in flatworms or nematodes. These features announced in some grade in all larger, more than circuitous animals:
- a coelom, a trunk crenel between the digestive tube and the external body wall that is lined with tissue
- a circulatory organization consisting of a series of tubes (vessels) filled with fluid (blood) to transport dissolved nutrients, oxygen, and waste material products apace and efficiently
Call up that the coelom is a fluid-filled crenel lying between the digestive tube and the outer torso tube and surrounded by mesodermal tissue. The digestive tube lies inside the outer torso tube. This arrangement is called "tube-within-a-tube construction" (Fig. 3.46). The fluid in the coelom supports the soft tissues of the body wall much every bit it does in the hydrostatic skeleton of cnidarians. Mesodermal muscles in the wall of the body tube and digestive tube can put force per unit area on the fluid to assistance in move. 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. iii.47). These contractions produce the crawling movement of worms. Retrieve that nematodes lack circular muscles, and can but move by contracting their longitudinal muscles, thus thrashing and wriggling rather than crawling. The setae along the body of polychaetes stick in the substrate, belongings parts of the worm in place while other parts motility frontward.
Annelids take a closed circulatory system in which blood is pumped along by muscles in claret vessels (Fig. 3.48). Blood flows through the microscopic capillaries, picking up food molecules from the digestive tract and oxygen from the pare and transporting them to the cells of the body. The parapodia, the flaps on the sides of the segments, increase the surface surface area of the skin for respiration. In an efficient circulatory system like this, an animal'southward internal tissues need not be close to its digestive and respiratory organs considering the claret delivers nutrients and oxygen. Such a organization lets animals abound much larger than possible in the flatworms, which must rely on diffusion.
The nervous organization is also more than complex in annelids than in other worm-similar phyla. Annelids have a simple brain organ consisting of a pair of nerve clusters in the head region (Fig. three.49). Nerves link the brain to sensory organs in the head that discover the environment in front of the worm. Earthworms are eyeless, but polychaete annelids accept eyes that tin can distinguish between light and dark. Some polychaete worm eyes tin fifty-fifty observe shapes. Nerves as well extend from the brain effectually the digestive tube and along the ventral surface. A ganglion or cluster of nervus cells operates the organs in each segment.
The excretory system of annelid worms consists of a pair of pocket-sized tubes in each segment. These tubes, chosen nephridia (from the Greek root word nephrus meaning kidney), are open at both ends. They filter coelomic fluid, which contains useful nutrient molecules forth with waste molecules. As the fluid moves through the tube, useful molecules render to the coelom, and waste matter molecules pass into the h2o. Although this system appears less circuitous than a flatworm's, nephridia are actually a more than efficient method of treatment waste material products considering they filter fluid, keeping useful molecules inside the trunk (Fig. iii.l).
Source: https://manoa.hawaii.edu/exploringourfluidearth/biological/invertebrates/worms-phyla-platyhelmintes-nematoda-and-annelida
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