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Worms: Phyla Platyhelmintes, Nematoda, and Annelida

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The content and activities in this topic will work towards building an understanding of the worms in the phyla Platyhelminthes, Nematoda, and Annelida.

Introduction to Worms

Most people are familiar with earthworms found in garden soil. Although many different kinds of animals are commonly lumped together as “worms,” there are several distinct phyla that fit the category. Worms are typically long, thin creatures that get around efficiently without legs. The different phyla of worms display a great range in size, complexity, and body structure. Flatworms (phylum Platyhelminthes) are simple animals that are slightly more complex than a cnidarian. Roundworms (phylum Nematoda) have a slightly more complex body plan. 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.



Fig. 3.35. (A) A whale shark (Rhincodon typus; a vertebrate animal)

Image courtesy of Tanaka Juuyoh, Flickr

Fig. 3.35. (B) A swimming polychaete worm (Tomopteris sp.; an invertebrate animal in the phylum Annelida)

Image courtesy of Uwe Kils, Wikimedia Commons


Worms are invertebrate animals with bilateral symmetry. Worms have a definite anterior (head) end and a posterior (tail) end. The ventral surface of worms and other organisms is the bottom side of the body, often closest to the ground. The dorsal surface is located on the upper part of the body 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 can detect the kinds of environment they encounter by moving in the anterior direction.


There are six features and systems that reveal an evolving complexity in the body structure of most worms:

  1. a mesoderm, an intermediate body layer between the inner (endoderm) and outer (ectoderm) tissue layers that forms muscle tissue
  2. a central nervous system guided by a “brain”
  3. an excretory system to eliminate some kinds of waste products
  4. a complete digestive system, from an anterior mouth to a posterior anus
  5. a coelom, a body cavity between the digestive tube and the external body wall that is lined with tissue
  6. a circulatory system consisting of a series of tubes (vessels) filled with fluid (blood) to transport dissolved nutrients, oxygen, and waste products around the body rapidly and efficiently

Flatworms: Phylum Platyhelminthes

The phylum Platyhelminthes consists of simple worm-like 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 word helminth meaning worm. Flatworms live 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 inside other animals—including humans—can injure or even kill the host organism. Free-living non-parasitic flatworms are typically less than 10 centimeters long. Marine species live buried in the sand or under rocks in shallow water. All free-living flatworms are predators that actively hunt 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. 3.36 B).



Fig. 3.36. (A) Free-living marine flatworm Maritigrella fuscopunctata

Image courtesy of Alexander Vasenin, Wikimedia Commons


Fig. 3.36. (C) Tapeworm Taenia saginata

Image courtesy of the Center for Disease Control (CDC)

Fig. 3.36. (D) Marine flatworm Pseudobiceros fulgor

Image courtesy of Christophe Cadet, Wikimedia Commons


Fig. 3.36. (E) Freshwater planarian flatworm Dugesia subtentaculata

Image courtesy of Eduard Solà, Wikimedia Commons

Fig. 3.36. (F) Yellow papillae flatworm (Thysanozoon nigropapillosum) swimming, Manta Ray Bay, Yap, Federated States of Micronesia

Image courtesy of Betty Willis, Wikimedia Commons



Fig. 3.16. Cross-sectional diagram of endoderm, ectoderm, and mesoderm tissue germ layers in diploblasts and triploblasts

Image by Narrissa Spies

Flatworms are more complex than cnidarians. Cnidarians have two layers of cells, the ectoderm and the endoderm; flatworms have a middle layer called the mesoderm between the other two layers (Fig. 3.16). 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 subsequently study have a mesoderm and muscular system. The cells of the ectoderm and endoderm are also more organized than similar cells of cnidarians. For the first time, we see groups of tissues that have evolved to form organs, such as the ones in the digestive, nervous, and excretory systems.



Fig. 3.37. Marine flatworm showing (A) dorsal view (B) cut away view of digestive system (C) Pharynx extended for eating in a cut away view (D) Pharynx retracted in a cut away view

Image by Byron Inouye

Like the cnidarians, flatworms have a digestive system with only a single opening into the digestive cavity, but in independently living marine flatworms the cavity branches into all parts of the body (Fig. 3.37 B). These flatworms feed through a pharynx. A pharynx is a long, tubular mouthpart that extends from the body, surrounds the food, 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 move to other cells of the body. Undigested food passes back out through the mouth, as in the cnidarians. Parasitic tapeworms usually absorb their nutrients directly from the host, while parasitic flukes have retained a digestive system.




Fig. 3.38. Nervous system of a planarian flatworm

Image by Byron Inouye

Like most self-propelling animals, independent-living flatworms have a central nervous system. A central nervous system consists of a mass of nerve cells, called a ganglion, (in more complex organisms, the ganglion evolves into a brain) in the anterior part of the body, and a nerve cord extending from the brain toward the posterior end of the body (Fig. 3.38). Sensory cells in the head detect changes in the environment. In free-living flatworms, sensory cells that respond to light are clustered in two eyespots in the head. Sensory cells that detect water currents, solid objects, and chemicals are in two flap-like projections on the head called auricles. In self-propelling animals, these sensory organs in the head are the first part of the animal that encounters new surroundings. The ganglion receives information from the sensory structures and sends signals to other parts of the body along 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 often called a “nerve ladder.”




Fig. 3.39. Excretory system of a planarian flatworm showing excretory pore, flame bulb, and flagella

Image by Byron Inouye

The excretory system removes waste products and excess water from tissues of flatworms. Flatworms have a surprisingly elaborate system to rid the body of wastes (Fig. 3.39). This network runs the length of the animal on each side and opens to the outside through small pores in the posterior region of the body. Connected to the tubes are tiny cells that move wastes and water 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. Under a microscope the flagellar movement looks like a flickering fire, and the structure is called a flame bulb.




Fig. 3.40. Arrangements of cell clusters (A) Cluster of cells in a sphere (B) Double-layered bag of cells (phylum Cnidaria) (C) Flat cluster of cells (phylum Platyhelminthes)

Image by Byron Inouye

Flatworms have no circulatory system. Animals without a circulatory system have limited 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 as they move away from their source. This is known as diffusion. A ball-shaped marine animal would not get adequate oxygen and nutrients to its innermost cells because the cells are too far from the body’s surface for molecules to move (diffuse) to them (Fig. 3.40 A). But cnidarians have no problem with diffusion because most cells of their bag-shaped bodies are in direct contact with the water, making the exchange of oxygen and nutrients easy (Fig. 3.40 B). Flatworms, bag-shaped but flattened, also get oxygen and nutrients to their body cells easily because all their cells are close to either their outer surface or their digestive cavity (Fig. 3.40 C). As animals become larger and more complex, diffusion is often no longer an option, and then we begin to see the development of circulatory and respiratory systems.

Roundworms: Phylum Nematoda

Species in the phylum Nematoda (from the Greek root word nema meaning thread) are better known as the roundworms (Fig. 3.41). There are about 25,000 species of nematodes formally described by scientists. Nematodes are found in almost every habitat on Earth. One species was first discovered living inside felt beer coasters in German alehouses. Studies of farmlands have found as many as 10,000 nematodes in 100 cubic centimeters (cm3) of soil. Nematodes are similarly abundant in marine and freshwater sediments where they serve as important predators, decomposers, and prey for other species like crabs and snails.



Fig. 3.41. (A) Parasitic hookworms (Ancylostoma caninum) in human intestinal tract

Photo courtesy of Centers for Disease Control and Prevention (CDC)

Fig. 3.41. (B) This animated image (click the image to see the animation) shows the typical crawling locomotion of nematodes. Caenorhabditis elegans is commonly used as a laboratory test model organism.

Photo courtesy of Bob Goldstein, Wikimedia Commons


Fig. 3.41. (C) Giant roundworm (Ascaris lumbricoides), the nematode parasite that causes the disease ascariasis in humans

Photo courtesy of Centers for Disease Control (CDC), Division of Parasitic Diseases

Fig. 3.41. (D) Pork worm Trichinella spiralis inside pig muscle tissue (under black pointer), the nematode parasite that causes the disease trichinosis in humans

Photo courtesy of froggerlaura, Wikimedia Commons



Fig. 3.41. (E) Rat lungworm (Angiostrongylus cantonensis), a nematode parasite that can cause meningitis

Photo courtesy of Lindo et al. 2002 originally published in Emerging Infectious Diseases

Like flatworms, roundworm species adopt either a free-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 have demonstrated a remarkable ability to remain dormant for decades until environmental conditions become favorable.


Like the flatworms, nematodes are bilaterally symmetrical. 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. Complete digestive systems are seen in more complex organisms and offer many advantages over the flatworm’s method of digestion. 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. 3.42). 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.



Image caption

Fig. 3.42. Typical regions of specialization in a complete digestive system

Image copyright and source

Image by Byron Inouye




Fig. 3.17. (A) Acoelom or lacking a fluid-filled body cavity (B) Coelom (C) Pseudocoelom

Image by Byron Inouye

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. 3.17 C).


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.

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 (class Hirudinea) are well-known parasitic members of the phylum, most commonly found in freshwater. The polychaete worms or “bristleworms” (class Polychaeta) are the largest group in the phylum Annelida. They occur mostly in marine and brackish water habitats.



Fig. 3.43. (A) Oligochaete; a species of Asian earthworm Amynthas sp.

Image courtesy of Michael Linnenbach, Wikimedia Commons

Fig. 3.43. (B) Medicinal leech (Hirudo medicinalis)

Image courtesy of Karl Ragnar Gjertsen, Wikimedia Commons


Fig. 3.43. (C) A paddleworm (Phyllodoce rosea) is an example of a motile or “errant” polychaete because its adult form uses muscles to move from location to location.

Image courtesy of Hans Hillewaert, Wikimedia Commons

Fig. 3.43. (D) Christmas tree worms (Spirobranchus spp.) live embedded in hard coral skeletons and are examples of sessile or sedentary polychaetes.

Image courtesy of Daniel Dietrich, Flickr


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. Figure 3.44 shows two examples of polychaete 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. 3.44 A). These bristles can penetrate human skin, causing irritation, pain and swelling, similar to the irritation caused by exposure to fiberglass.



Fig. 3.44. (A) A bearded fireworm Hermodice carunculata

Image courtesy of National Oceanic and Atmospheric Administration (NOAA), National Marine Sanctuaries


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. 3.45). 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.



Fig. 3.45. (A) Ice cream cone worm, Pectinaria koreni with and without tube (Family Pectinariidae)

Image courtesy of Hans Hillewaert, Wikimedia Commons

Fig. 3.45. (B) Feather duster worm (Sabellastarte australiensis) in a coral colony

Image courtesy of John Turnbull, Flickr


Fig. 3.45. (C) Sand mason worms (Lanice conchilega) build straight tubes using sand grains and shell fragments.

Image courtesy of Matthias Buschmann, Wikimedia Commons

Fig. 3.45. (D) Sand mason worm (Lanice conchilega) without its tube

Image courtesy of Matthias Buschmann, Wikimedia Commons


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:

  1. a coelom, a body cavity between the digestive tube and the external body wall that is lined with tissue
  2. a circulatory system consisting of a series of tubes (vessels) filled with fluid (blood) to transport dissolved nutrients, oxygen, and waste products rapidly and efficiently



Fig. 3.46. Cross-sectional diagram of a polychaete annelid worm showing the tube-within-a-tube construction of a true coleom body cavity

Image courtesy of Hans Hillewaert, Wikimedia Commons

Fig. 3.47. Contraction of muscles and movement in an earthworm

Image by Byron Inouye


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. 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 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. 3.47). 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.



Fig. 3.48. Circulatory system of a polychaete worm

Image by of Byron Inouye

Annelids have a closed circulatory system in which blood is pumped along by muscles in blood vessels (Fig. 3.48). 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. In an efficient circulatory system like this, an animal’s internal tissues need not be close to its digestive and respiratory organs because the blood delivers nutrients and oxygen. Such a system lets animals grow much larger than possible in the flatworms, which must rely on diffusion.



Fig. 3.49. Nervous system of a polychaete worm

Image by Byron Inouye

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. 3.49). 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. A ganglion or cluster of nerve cells operates the organs in each segment.



Fig. 3.50. Excretory system of a polychaete worm

Image by Byron Inouye

The excretory system of annelid worms consists of a pair of small tubes in each segment. These tubes, called nephridia (from the Greek root word nephrus meaning kidney), are open at both ends. They filter coelomic fluid, which contains useful nutrient molecules along with waste molecules. As the fluid moves through the tube, useful molecules return to the coelom, and waste molecules pass into the water. Although this system appears less complex than a flatworm’s, nephridia are actually a more efficient method of handling waste products because they filter fluid, keeping useful molecules inside the body (Fig. 3.50).


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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.