flatworms

flatworms

The word "worm" is not very scientific. But it is a word that informally describes animals (usually invertebrates) that have long bodies with no arms or legs. (Snakes are vertebrates, so they are not usually described as worms.) Worms are the first significant group of animals with bilateral symmetry, meaning that the right side of their bodies is a mirror of the left. One type of worm is the flatworm. Worms in the phylum Platyhelminthes are called flatworms because they have flattened bodies. There are more than 18,500 known species of flatworms.

features of flatworms

The main characteristics of flatworms ( Figure 1.1) include: 1. Flatworms have no true body cavity, but they do have bilateral symmetry. Due to the lack of a body cav- ity,flatworms are known as acoelomates. 2. Flatworms have an incomplete digestive system. This means that the digestive tract has only one opening. Digestion takes place in the gastrovascular cavity. 3. Flatworms do not have a respiratory system. Instead, they have pores that allow oxygen to enter through their body. Oxygen enters the pores by diffusion. 4. There are no blood vessels in the flatworms. Their gastrovascular cavity helps distribute nutrients throughout the body. 5. Flatworms have a ladder-like nervous system; two interconnected parallel nerve cords run the length of the body. 6. Most flatworms have a distinct head region that includes nerve cells and sensory organs, such as eyespots. The development of a head region, called cephalization, evolved at the same time as bilateral symmetry in animals. This process does not occur in cnidarians, which evolved prior to flatworms and have radial symmetry. Marine flatworms can be brightly colored, such as this one from the class Turbel- laria. These worms are mostly carnivores or scavengers.

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flatworms in the environment

Flatworms live in a variety of environments. Some species of flatworms are free-living organisms that feed on small organisms and rotting matter. These types of flatworms include marine flatworms and freshwater flatworms, such as Dugesia. Other types of flatworms are parasitic. That means they live inside another organism, called a host, in order to get the food and energy they need. For example, tapeworms have a head-like area with tiny hooks and suckers (known as the scolex) that help the worm attach to the intestines of an animal host ( Figure 1.2). There are over 11,000 species of parasitic flatworms.

instructional diagrams

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questions

the flatworms are the first significant phylum with bilateral symmetry.

-->  a. true

b. false

most flatworms are parasites.

-->  a. true

b. false

how does respiration occur in flatworms?

a) respiration occurs by the movement of oxygen over the gills.

b) respiration occurs by the diffusion of oxygen into the lungs.

-->  c) respiration occurs by the diffusion of oxygen into the worm body.

d) all of the above methods are used by worms to respire.

what is cephalization?

-->  a) cephalization is the development of a head region.

b) cephalization refers to the flatworms head region.

c) cephalization includes the two eye spots.

d) cephalization is the development of the nerve net running the length of the flatworm body.

how are nutrients distributed throughout the flatworm?

a) nutrients are carried by the blood through the flatworm.

b) nutrients are distributed by diffusion.

c) nutrients are distributed by the digestive tract.

-->  d) nutrients are distributed by the gastrovascular cavity.

which best describes the flatworm digestive system?

a) flatworms have a complete digestive system, with two openings, a mouth and an anus.

b) flatworms have an incomplete digestive system, with two openings, a mouth and an anus.

-->  c) flatworms have an incomplete digestive system, with just one opening.

d) flatworms do not have a digestive system.

a body cavity usually refers to the space located between an animals outer covering and the outer lining of the gut cavity. coelomates have a complete body cavity where organs attach and develop. pseudocoelomates have a body cavity that loosely holds organs in place. acoelomates do not have a body cavity. what type of animals are flatworms?

-->  a) coelomates

b) pseudocoelomates

c) acoelomates

d) none of the above

diagram questions

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