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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.
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.
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.
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.
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.
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.
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).
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.
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.
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:
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.
Worm composting is using worms to recycle food scraps and otherorganic material into a valuable soil amendment called vermicompost,or worm compost. Worms eat food scraps, which become compost asthey pass through the worm's body. Compost exits the worm throughits' tail end. This compost can then be used to grow plants. Tounderstand why vermicompost is good for plants, remember thatthe worms are eating nutrient-rich fruit and vegetable scraps,and turning them into nutrient-rich compost.
For millions of years, worms have been hard at work breakingdown organic materials and returning nutrients to the soil. Bybringing a worm bin into the classroom, you are simulating theworm's role in nature. Though worms could eat any organic material,certain foods are better for the classroom worm bin.
Setting up a worm bin is easy. All you need is a box, moistnewspaper strips, and worms. To figure out how to set up a wormbin, first consider what worms need to live. If your bin provideswhat worms need, then it will be successful. Worms need moisture,air, food, darkness, and warm (but not hot) temperatures. Bedding,made of newspaper strips or leaves, will hold moisture and containair spaces essential to worms.
You should use red worms or red wigglers in the worm bin, whichcan be ordered from a worm farm and mailed to your school. Thescientific name for the two commonly used red worms are Eiseniafoetida and Lumbricus rubellus.
When choosing a container in which to compost with worms, youshould keep in mind the amount of food scraps you wish to compost,and where the bin will be located. A good size bin for the classroomis a 5- to 10- gallon box or approximately 24" X 18"X 8". The box should be shallow rather than deep, as redwigglers are surface-dwellers and prefer to live in the top 6"of the soil..
If you take care of your worms and create a favorable environmentfor them, they will work tirelessly to eat your "garbage"and produce compost. As time progresses, you will notice lessand less bedding and more and more compost in your bin. After3-5 months, when your bin is filled with compost (and very littlebedding), it is time to harvest the bin. Harvesting means removingthe finished compost from the bin. After several months, wormsneed to be separated from their castings which, at high concentrations,create an unhealthy environment for them.
Over the next 2-3 weeks, the worms will move over to the newside (where the food is), conveniently leaving their compost behindin one section. When this has happened, remove the compost andreplace it with fresh bedding. To facilitate worm migration, coveronly the new side of the bin, causing the old side to dry outand encouraging the worms to leave the old side.
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