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There are about thirty thousand species of worms in the phylum Platyhelminthes. Twenty five thousand of these worms are parasitic, which is the vast majority of the phylum. The flatworms only account for four thousand of the worms in the phylum, but they are said to be very beautiful compared to the parasitic worms (Snyderman and Wiseman 83). Flatworms display bilateral symmetry, which means that their bodies have a distinctive head and rear, and their right and left sides are mirror images of each other. The fact that these worms are bilaterally symmetric enables them to move from place to place more efficiently. This trait can help the worms catch their food and get away from their enemies. It can help them find a mate and move from location to location. If they were not made like this, they would have very little control over their bodily movements, and not be able to survive in the ocean (Snyderman and Wiseman 84).
Some other characteristics that make worms of the phylum Platyhelminthes unique are the fact that they are light sensitive. They often try to avoid brightly lit areas. They are so sensitive because they possess’ bundles of light-sensitive eyes that are often described as primitive eyes (Snyderman and Wiseman 84). Another interesting characteristic of the flatworms is that they are hermaphroditic. This means that they posses both male and female reproductive organs. When one learns of this they may think that self-fertilization would be common, but it is not. Flatworms also posses amazing regenerative properties. When they lose a body part, a new one will form, and it some cases, the body part often regenerates an entirely new flatworm (Snyderman and Wiseman 84).
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One can find Marine flatworms in shallow rubble zones. They like to crawl in and under the rubble. Sometimes, the rubble and debris will rip or tear the flatworm’s skin, but because of their amazing regenerative properties, this is not much of a problem. Flatworms are usually spotted on the bottom of the ocean, but some species have been found swimming. They can do this by using an undulating motion of their body. This is due to the fact that they are bilaterally symmetrical (Meinkoth 401).
There are three classes included in the phylum Platyhelminthes. They are the Turbellaria, Trematoda, and the Cestoidea. The turbellaria class includes mostly free –living forms. This means that they are not parasitic and do live off another organism. The most well known members of the Turbellaria class are the Tricladida and Polycladida.
These are the members that are most commonly found on the beach (National Audubon Society 399). Triclads have a digestive cavity that contains three major branches. They lay their eggs in a solid bubble that they then attach to an immovable object. Development of the eggs is direct in triclads. The young worms are tiny replicas of adult worms. Unlike the triclads three major branches, polyclads have numerous branches in their digestive cavity. Also dissimilar is the eggs are not enclosed in a shell. The development can be direct in some species and indirect in others. In indirect development, the larva later goes metamorphosis into adult form (National Audubon Society 401).
An example of a marine worm in the order Tricladida is the limulus leech. This worm can be found from the Gulf of Maine to Florida and Texas. This worm is very active and can attach its sucker to a horseshoe crab. By doing this, it feeds on particles brought in by the host. When the worm is removed from the crab, it actually shrinks in size and will not feed anymore. The limulus leech lays its eggs enclosed in a capsule attached to the tissue of the horseshoe crabs gills by a strong stalk. You can identify this worm by its elongated body and its narrow head. It is white or light yellow, and has brown intestines. The limulus leech’s mouth is on the underside and this is where its long sucker is attached. The sucker is its feeding tube. This worm averages a size of about sixteen millimeters long and six millimeters wide (National Audubon Society 400).
An example of a marine worm in the order Polycladida id the Oyster leech. This worm can be found in the Bay of Fundy and Florida and Texas. Oyster leeches attach themselves to open oysters and feed on their soft tissue. This worm is also known to feed on barnacles. Because of their feeding habits, the Oyster leech lives in oysters and barnacles. These worms have the characteristics of being very thin and flat. They also have an oval shape to them. They are pale yellow and often have a brown stripe down the center of their back. They have two retractable tentacles and two clusters of eyespots over their brain. Their mouth is also on the underside of their bodies. Their average size is twenty-five millimeters long and ten millimeters wide. There is a species called the red oyster leech that is slightly larger than the regular oyster leech. This also ranges from Florida to Texas, and is red with pink spots (National Audubon Society 401).
Another common order of the Polyclads is the tapered flatworm. This worms is one of the most common flatworms on rocky shores. It is also known to be an aggressive predator and eats animals half of its size. Like many other marine worms, most specimens are hermaphroditic. This species also contains the speckled flatworms, which is the most common flatworm on the rocky coast of New England. This worm can be found on the Coast of California, and it lives between and under rocks. The tapered flatworm can be pale gray or tan in color and it has darker spots around its middle section. Something interesting about this worm is, when the worm is full of food; the branches of its digestive track are actually visible through the skin. This worm averages in size of about sixty millimeters in length and about nineteen millimeters in width. This worm has no obvious tentacles but has round clusters of eyespots. In fact it has twenty-five eyespots in long bands on each side of the head over the brain. The tapered worm is flat and it is noticeably oval (National Audubon Society 402).
Now that we have reviewed the phylum platyhelminthes, we are now going to look at the ribbon worms that belong to the phylum nemertea. The nemerteans are unsegmented worms. They have soft bodies and often have few easily observed external diagnostic features. The way taxonomists make accurate identification is through their internal anatomy and history (nemertea from San Francisco Bay). The sizes of these different species vary greatly. The smallest species is one millimeter long, but there are larger species that can extend to up to 30 meters in length. An interesting fact about these worms is that they are extremely flexible and can lengthen their bodies up to five times their minimum length. This means that taxonomists can only estimate their true size (nemertea from San Francisco Bay).
The nemerteans general shape is its most distinctive feature. Is general shape is said to be vermiform which means worm shaped. It has an organ that lies in an anterior internal cavity called the rhynchocoel. This organ is shot out from the mouth when the animal is hunting, defending itself, or moving. Most species of nemerteans are carnivorous. They eat small invertebrates such as annelids and crustaceans and the eggs of these invertebrates. Other species of these worms live symbiotically in the shells of mollusks and feed on plankton filtered by their host. There are however omnivorous species, which eat most plants and other invertebrates (Nermeteans from San Francisco Bay).
The gonadel systems of this worm are simple and reproduction involves generating by separate sexes. A few species have been known to exhibit internal fertilization. These worms are also good at regenerating a body part that has fallen off. There are about one thousand know nemerteans, and scientists say there are several times this number that have not been discovered yet. There are a few worms living in fresh water and many living in the ocean. This group however is not very well known and they live hidden lives with few scientists studying them (Nemerteans from San Francisco Bay).
An example of a nemertean worm is the proboscis worm, parvorlasia corrugatus. It is found in the following places: Antarctic peninsula, the South Shetland Islands, South Orkney, the South Georgia Islands, the Kerguelen island, Cargados in the Indian ocean, Tierra del Fuego, Southern Argentina, Peru, and Chile. This worm has a flattened body and it grows to length of one or two meters. It has a very big appetite and eats almost anything. It is considered a fierce scavenger and predator and that is how it gets its food. Feeding pile-ups of up to twenty-two worms have been observed by scientists. Because of its chemotactic sense it can detect food at a distance and its large mouth can engulf food almost as large as itself (Nemerteans from San Francisco Bay).
These worms lack a respiratory system so they have to absorb oxygen through the skin. The oxygen can be soaked up so easily because of the low metabolic rate of these creatures and because of the cold Antarctic waters. If the oxygen level drops in the water the worm becomes more flattened and elongated, which facilitates oxygen uptake by increasing its skin area and also minimizes the distance that oxygen must diffuse into its body (Nemerteans from San Francisco Bay). These worms have a closed circulatory system and a one-way gut. They are the simplest animals with a circulatory system. Their wedge shaped head has a fluid filled cavity used to shoot a barbed proboscis very fast, which they use to get their prey or to protect them. This harpoon like feature has adhesive secretions, which secure prey making sure they do not get away from the worm (Nemerteans from San Francisco Bay).
As stated earlier, the high concentration of oxygen is responsible for the worms growing to giant size. This is directly correlated to the coldness of the water. Nemertean worms found in temperate waters only grow to ten percent or less of the size of the Antarctic ribbon worm (The New Zealand Institute.) Because of this mechanism in the worms relates to temperature Professor Davison from the New Zealand Antarctic Institute says, “These worms may be an excellent indicator of global climate change due to their extreme sensitivity to small changes in their environment. They are fascinating creature ranging in colors from red, yellow, brown, and white with a variety of patterns on their skin.” Currently, Professor Davison is conducting several experiments using these worms. His primary goal is to look at the effects of small temperature increases, and changes in oxygen on the metabolic rate and survival of the animal. These worms are oxyconformers. This means that they need fully oxygenated water in order to meet their oxygen needs. It is so important that even small doses in these oxygen levels could be harmful to these worms’ metabolic rates (The new Zealand Antarctic Institute).
Now that we are done with the nemertea, we are going to explain about the nematodes. Nematodes are one of the most common phyla of animals, with over 20,000 different species. They are able to thrive in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts. There are also quite a few parasitic forms, including pathogens in most plants and animals, humans included (“Wikipedia”).
Nematodes are roundworms that are bilaterally symmetrical with the same organisms as worms. A flexible, non-cellular layer called the cuticle surrounds their body structure. A layer of skin cells protects the cuticle. Under the skin by the body wall lies the muscle cell. The muscles go in longitudinal directions only. The nematode worms
have a pseudocoel instead of a coelom. The pseudocoel is formed directly from the cavity of the blastula. It is a small cavity that is filled mostly with intestine, testes, or oviducts. The nervous system is also very simple, consisting of a ring of nervous tissue around the pharynx, which makes the dorsal rise, and the ventral nerve cord that runs down the body (“The University of Michigan-Museum of Zoology”).
The way nematodes move is by contraction of the longitudinal muscles that thrash back and forth. They have high internal pressure that makes their body flex rather than flatten. Nematodes have no cilia or flagella found on their body. Most nematodes are dioecious. When fertilization takes place in a nematodes society, the males use a special copulatory spine to open the female’s reproductive tracts to insert them with their sperm. The male sperm also lack flagella and they move by pseudopodia, similar to amoebas. The development of fertilized eggs in nematodes is almost always direct (“The University of Michigan-Museum of Zoology”).
Most free-living (non-parasitic) nematodes are so small they can only be seen through a micrscope, though a few parasitic forms can grow to be several meters long. There are no circular muscles, so the body can only move from side to side. In order to
actually get anywhere, the worm needs to be in contact with solid objects. Different species feed on materials differing from algae, fungi, small animals, fecal matter, dead organisms and living tissues (“Wikipedia”.)
Nematodes may also infect many different species of wild fish. Small numbers of nematodes often occur in healthy fish, but high numbers will cause illness or even death in these fish. In aqua culture systems, the fish that are infected with a small number of nematodes may not even show signs of illness, but often have reduced reproductive rates. However, younger fish infected by small numbers of nematodes are more likely to show signs of illness and also have reduced rates of growth (“University of Florida”). Adult nematodes are commonly found in the digestive tracts of fish. However, depending upon the species of nematode and the species of infected fish; adult nematodes can be found in several different parts of the fish, including the internal organs, and the swim bladder. They may also be found in the outside muscle layers and layers of the skin and fins (“University of Florida”).
The type of disease in a fish depends on five factors. The factors are the life stage, the species, the number of nematodes that are in the fish, the age and species of infected fish, and the places of infection. The nematodes can thrive anywhere, but they are most commonly present in muscle, the liver, and tissues surrounding the internal organs. You will be able to tell if a fish is infected if it is hemorrhaging, or if it has cysts. Lumps on the outside of the fish may also mean a nematode is living in the fish. The nematodes in the fishes’ intestines cause an eating away at the fish’s nutrients causing it to waste away (“The University of Florida”.)
We have now discusses three out of six phyla of marine worms: the
Platyhelminthes, the Nemertea, and the Nematode. We have learned that other than their worm-like shape, there is not much that they have in common. When compared to other animals, worms appear to be simple creatures, but they have so many differences that the scientists had to divided them among several phyla. They are a very diverse and unique group of animals that inhabit our ocean floor.