Worms 2BIbDF09 10/11

What is a worm ?

Worms are invertebrate animals, haven't a skeleton. They are multicellular with true tissues and bilateral symmetry and they have a soft and flexible body. [They are protostomes.] They probably appeared at the beginning of cambrian era and they are an important step of animals' evolution. There are several Phyla and each one live in a specific environment. Some of them are parasitic organisms that can live in a body or just live in soil or in the sea.

(See appendix 1 : Roundworm / Flatworm / Segmented Worm)

How are they classified ?

Worms are separate in three classes, depending on their body shape :

There are three basic kinds of body : acoelomate, which hasn't any body cavity; pseudocoelomate, which has false body cavity located between the mesoderm and endoderm; and coelomate, which has fluid-filled true body cavity that develops entirely in the mesoderm. (See appendix 5)
Flatworms are Acoelomates, Roundworms are Pseudocoelom and Segmented Worms are Coelomate. Most members of the animal kingdom are coelomates. The gut and other internal organs are suspended within the coelom. The advent of this type of body cavity necessitated the development of a more complex circulatory system to ensure that all organs receive oxygen and nutrients.

- Flatworms or Platyhelminthes
That contain the :

• Turbellaria
• Cestoda

• Trematoda
• Monogena

- Roundworms or Nematoda
That contain the :

• free-living worms
• Parasitic worms

- Segmented Worms or Annelida

That contain the:

• Polychaetes
• Clitellates (including eathworms and leeches)

What are the main characteristics of each one?

Flatworms or Platyhelminthes

General physical characteristics: Platyhelmintes are flat and they can measure about 1mm to 20m in length, their are the simplest animals with bilateral symmetry. They have no distinctive head or anus but they have a single opening that serve them to eat and to defecate. Some parasitic species have a Head called scolex and constituted by hookers and suckers. They are billaterians but they neither have body cavities, nor respiratory or circulatory organs. And that limits their sizes and their body shapes, because oxygen has to arrive and carbon dioxide to leave their body by diffusion. Hence, nearly all the Plathyelminthes are microscopic or have flat ribbon-like or leaf-like shapes. Platyhelminthes breathe by the whole surface of their body and so they are vulnerable to fluid loss, and that's why they have to live in freshwater, in the sea or in moist environments because they are not subject to dessication.

There are about 20'000 species of flatworms

Where are they in the tree of life?

• Domain : Eukaria
• Kingdom: Animalia
• Phylum: Platyhelminthes
• Class: Turbellaria, Cestoda, Trematoda, Monogenea

The Phylum of Platyleminthes includes four sub-groups:

• The Turbellaria, they are mostly non-parasitic organisms, they live in water or in moist environnements. They can mesure from 1mm to 60cm and they are mostly all predators or scavengers and hermaphrodites. Turbellaria are acoelomates,as all the Platyleminthes. Turbellarians move using cilia on their ventral surfaces or use their muscles to swim trough water, with an undulating motion. The best-known turbellarians are the Plannarians, they can reproduce asexually trough fission, but they also can reproduce sexually and because they are hermaphrodites, copulating mates typically cross-fertilize each other.

• The Cestoda, or tapeworms are parasitic organisms. They mostly live in vertebrates intestines. Their head(scolex) is constituted by a sucker and a hook that allow them to attach to the intestinal wall. The tapeworm grow and feed itself absorbing and digesting the nutrients that the host ingest. The nutrients pass trough it's skin and the tape grows, when it's growing it forms segments and in each segment there is an independent reproduction and digestion system. Each segment is known as a proglottid, and contains both male and female reproductive organs, capable of bearing fertilized eggs.

• • A Taenia life-cycle: The cycle begin when a Taenia larval cryst is ingested by somebody, who ate poorly cooked meat or fish. Then the larva pass in the intestine, and when it's in the intestine it attach itself to the intestinal wall by it's hooks and and it's suckers and it start to create proglottid(segments). In theses segments a mature worm will grow in 3-4 months and produce a chain of other proglottid. When the proglottids are mature their are released in the feces and then they will contaminate the soil and the water. Then they will be eaten or drunk by an animal and they will develop themselves in it's intestine and the cycle will start again.
    

see appendix 2 (taenia life cycle)

• The Trematoda, or Flukes are parasitic organisms. They mostly live in mollusks but also in fish or humans. They nourish themselves ingesting the food that their host eat, by fixing themselves with their suckers in their host intestines. They are hermaphrodites and they reproduce in their host by sexual reproduction. Flukes have a hard protection skin that protect them from being dissolve by stomach acids. There are about a 8 to 24 thousand species of Flukes.

• The Monogenea, they are small parasitic organisms (from 1 mm to 2 cm) and they mostly live in skin or gills of fish. They attach their host with their hooks and feed eating the blood or the skin of their hosts. Monogena are hermaphrodites but they can fertilize each other, they live in the same host for their entire life cycle. The fertilized eggs produce larvae, that are released in the water, they have to swim to another host to nourish themselves, that means that their survival chances are limited to few hours.

Roundworms or Nematoda

Where are they in the tree of life?

• Domain: Eukaria
• Kingdom: Animalia
• Phylum: Nematoda

General physical characteristics : (see appendix 3 : Nematoda) Roundworms or Nematodes are unsegmented, round, slender, and they measure typically less than 2.5 mm long; The smallest round worm is microscopic, but free-living species can reach as much as 5 mm and some parasitic species can be larger.
Their body have ridges, rings, warts, bristles or other distinctive structures and it is bilaterally symmetrical. They have body cavity, which is surrounded by the muscle layer and the gut furrows the center of the cavity. Their head is relatively distinctive and contrary to the body, the head is radially symmetrical with sensory bristles and, in many cases, solid head-shields outwards around the mouth. Finally the mouth has three or six lips, which often have a series of teeth on their inner edge. Nematodes have a complex nerve cord, a well-developed digestive system and a complete reproductive organs.
They are found in most aquatic habitats, in wet soil, and as parasites in the body fluids and tissues of plants and animals.

Nervous system : Four nerves run the length of the body on the dorsal, ventral, and lateral surfaces. The dorsal one is responsible for motor control, the lateral one are sensory and the ventral one combines both functions.

Digestive system : The oral cavity, which is lined with cuticle, opens into a muscular sucking pharynx, also lined with cuticle. The pharynx is connected directly to the intestine that forms the main length of the gut and the gut. The last portion of the intestine has cuticle, which form a rectum, that expels waste through the anus.

Reproduction system : Nematodes are dioecious, that means that they are separated, like humans in two parts : male and female individuals. The male has testes, vas deferens, seminal vesicle and an ejaculatory duct. And the female has ovaries, oviduct, seminal receptacle, uterus and vagina. The reproduction is sexual by internal fertilization and usually the male is smaller and lower than the female. During copulation, one or more chitinized spicules move out of the cloaca, then they are inserted into genital pore of the female and the sperm crawls along the spicule into the female worm. But there are many modes of reproduction : some Nematodes use a process called "endotokia matricida", that causing maternal death and some ones are hermaphroditic, which keep their self-fertilized eggs inside the uterus until they hatch.

Nematoda includes two sub-groups:

• Free-living specie : Their development usually consists of four molts of the cuticle during growth and they have an important role in the decomposition process. They aid in recycling of nutrients in marine environments and are sensitive to changes in the environment caused by pollution.

• Parasitic specie: There are 50 parasitic roundworms species infect humains, include ascarids(ascaris), filarias, hookworms, pinworms (Enterobius, whipworms (Trichuris trichiura) and trichina worm (Trichinella spiralis) that causes trichinosis. Baylisascaris usually infests wild animals but can be deadly to humans as well. Some parasitic worms can be considerate by humans as beneficial because they parasitize insects. Depending on the species, a nematode may be beneficial or detrimental to plant health. From agricultural and

horticulture perspectives, there are two categories of nematode: predatory ones, which will kill garden pests like cutworms, and pest nematodes, like the root-knot nematode, which attack plants and those that act as vectors spreading plant viruses between crop plants.

• • the Trichinosis is caused by the Trichinella roundworms, or commonly known as trichina worm. Humans may acquire the parasite by eating infected, undercooked pork. The worms enter into the human intestine and can eventually travel to other parts of the body, invading muscles and other organs. Symptoms of trichinosis include fatigue, diarrhea, and fever. In severe cases, trichinosis can be fatal.

But some worms can be both free-living and parasitic at times.

Segmented Worms or Annelids

Annelids are worms with body segmentation, which is the division of the body in a series of repeated segments. There are about 17'ooo species of segmented worms, that are subdivided in two major sub-groups: The Polychaetes and the Clitellates. They mostly live in tidal zones,in freshwater or in moist terrestrial environments.

Where are they in the tree of life?

• Domain: Eukaria
• Kingdom: Animalia

• Phylum: Annelida
• Class: Polychaetes, Clitellates

General physics characteristics
Segemented worms have a segmented body, that's looks like a set of fused rings and all the segments are filled with the same set of internal organs, except the frontmost and the rearmost section, that are not considerate as true segments. The body of a segmented worm is composed by six lawyers by segment, from the external to the internal: The skin(or cuticle), the circular muscle, the longitudinal muscle, the peritoneum(that contain the nerves cord and the blood vessels), the pair of coelums ( a fluid filled true body cavity), and the guts see appendix 4.

Nervous system Annelids have a pair of nerve cord that is in their inferior part, they also have a brain. Each cell of their body is controlled by few neurons. They also have sensors, cells that detect: light, chemical products, pressure and contact. Most Annelids, including Polychaetes and Cittelates can regenerate after damages. Especially two species of Polychaetes can regenerates themselves from only one segment and others

Reproductive System Annelids can reproduce sexually or assexually. The assexual repoduction occurs by when dividing themselves into two or more pieces or by budding off a new individual while the parent remains a complete organism. When they reproduce sexually, Annelids produce male and female gametes, then the following egg give rise to a round larva called Trochophore, who lives like as a plankton. Then, later the larvae will sink to the sea floor and transform themselves into small adults.

What are their part in the body features' evolution?

Worms are the third step in the body features' evolution:

First there are sponges; they are multicellular and have cell layers but lack true tissues like nervous tissues.
Then there are Cnidarians, which have in addition a radial symmetry body.
Next there are Worms with no body cavity like Flatworms and then Roundworms, which have pseudocoelom and finally there are Mollusks and Annelids with a true coelom from cell masses.
Worms are very important in the body features' evolution because they were the first animals to have body cavity and coelom. First they were the Flatworms with a single opening, so an incomplete digestive tract like a tube, they lacked circulatory system to transport oxygen and food. Then they were the Roundworms(Nematoda)they were the first to have a complete digestive tract with a digestive tube and two openings: a mouth and an anus. And finally were the Annelids that were the first to have body segmentation, building a body from a series of repeated units. All animals, which come after the Worms have cavities, coelom and a digestive system.

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