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.

How are they classified ?

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

- 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.

Where are they in the three 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.

• 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
• The Monogenea, they are small parasitic organisms and they are hermaphrodite

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 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
• Parasitic specie

( ** Ascarids, Hookworms, Stongyloids, Heartworms, and Whipworm ) But some worms can be both free-living and parasitic at times.

Segmented Worms or Annelida

- Earthworms- Polychaetes- Leeches

What are their part in the body features' evolution?

very disappointing... we started this wiki on Nov29... Be careful... you should now produce a big amount of work in order to have enough information on your page and for you oral presentation Pierre.brawand 16 janvier 2011 à 16:02 (CET)

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