Worms 2BIbDF09 10/11

What is a worm ?

Worms are invertebrate animals. 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.

maybe a Appendix document here?Pierre.brawand 17 janvier 2011 à 11:38 (CET)

How are they classified ?

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

OK for this criteria. Indeed, the body shape is a good criteria. But you will also see that the coelom plays an important role in how to classify worms.Pierre.brawand 17 janvier 2011 à 11:38 (CET)
- Flatworms or Platyhelminthes
That contain the :

• Turbellaria
• Cestoda

• Trematoda
• Monogena

- Roundworms or Nematoda
That contain the :

• Hookworms
• Pinworms

- Segmented Worms or Annelida

That contain the:

• Earthworms
• Polychates

• Leeches

What are the 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 billateral symmetry. They have no distinctive head or anus but they have a single opening that serve them to eat and to defecate. The 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 Platyleminthes are microscopics or have flat ribbon-like or leaf-like shapes. Platyhelminthes breathe by the whole surface of their body and so they are vulerable to fluid loss, and that's why they have to live in freshwater, in the sea or in moist environnements 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 swimm trough water, with an undulating motion. The best-known turbellarians are the Plannarians, they can reproduce assexually trough fission, but they also can reproduce sexually and because they are hermaphrodites, copulating mates typically crossfertilize each other.

• The Cestoda, or tapeworms are parasitic organisms. They mostly live in vertebrates intestines. Their head(scolex) is constitued 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 segements and in each segment there is an independant 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: A Taenia larval cryst is ingested by somebody eating poorly cooked meat or fish, then it pass in the intestine. Then it attach itself to the intestinal wall by the hooks and the suckers and it start to create proglottid(segments)where in 3-4 months a mature worm will grow and produce a chain of other proglottid. When the proglottids are mature their are releases in the feces and may be eaten by a pork or an other animal an the cycle will start again.

• The Trematoda, or Flukes are parasitic organisms. They mostly live in mollusks but also in fish or humans. They nourrish 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. Ther 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 fetrtilize 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 swimm to another host to nourrish themselves, that means that their survival chances are limited to few hours.

Roundworms or Nematoda

General physical characteristics : Roundworms or Nematodes are unsegmented, round, slender, and they measure typically less than 2.5 mm long; The smallest one 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 centre 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.

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: Nematodes commonly parasitic on humans include ascarids(ascaris), filarias, hookworms, pinworms (Enterobius) and whipworms (Trichuris trichiura).

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

You should think about generating some Appendix documents in order to illustrate your text with figures.Pierre.brawand 30 janvier 2011 à 15:11 (CET)

Segmented Worms or Annelida

- Earthworms- Polychaetes- Leeches

What are their part in the body features' evolution?

much better... Think about generating Appendix docs with figures.Pierre.brawand 17 janvier 2011 à 11:38 (CET)

---

back to Animals_2BIbDF09_10/11
back to Accueil