Platyhelminthes

Diagnostic Characteristics:
Platyhelminthes, more commonly known as the flat worm, is very thin and between the dorsal and ventral surfaces. The sizes of these organisms vary greatly ranging from about 20 meters to being practically microscopic. These triploblastic animals used to appear to have a complex structure, however, compared to common animals the one opening in their gastrovascular cavity is rather simple. Flatworms lack a digestive tract so they must absorb nutrients through the surface of their body. The organism can be sorted into four different classes: tubellaria, monogenea, trematoda, and cestoidea.

Class Turbellarian live in fresh or marine water environments and either feed as scavengers or on protists. Class Trematoda are characterized as being parasites that live in a primary host where sexual reproduction would occur or a intermediate host where asexual reproduction would occur. Class Cestoda are long, flat tapeworms that live in intestines. These tapeworms use a structure called a scolex which is made of suckers and hooks as an adaptation to living in the intestines of a host (SD) (6).

The classes Turbellarian and Trematoda are paraphyletic which means that the group of organisms have a common ancestor but not all organisms in the group are necessarily descendents of that common ancestor. (AC)(8)

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Food and DIgestion:
The different classes of this organism have different ways of aquiring and digesting there food. The tubellaria group is made up of carnivores that eat animals that are either smaller than them or already dead. The gastrovascular cavity branches off to different parts of the organisms body and distributes the food. The organism then excretes ammonia, nitrogenous wastes, into the water surrounding it. Monogenea and trematoda are both parasites that attach to the internal organs or outside surface of their host. Cestoidea are also parasites, however they tend to latch onto vertebrae including humans and tend to lock onto the intestinal lining of their host. It lacks a digestive track so it absorbs food already digested by the host organism. A flatworm’s digestive and excretory system are closely related. Flatworms eat food and eliminate wastes through the same opening. (DB) (3) "Flame cells are the simplest forms of an excretory system to remove waste material from body, functioning like kidneys. Flame cells are found in bundles called protonephridia. In addition to removal of wastes, flame cells also regulate the osmotic pressure of the flat worm, and maintain its ionic balance. Microvilli found in the tube cell are used to reabsorb some important ions needed by the organism." (AK) (5)

Reproduction:
Flat worms can be both sexually and asexually reproductive. Many of them require and intermediate host so the larvae can develope. The larvae needs to fully develope before they can infect a host completely.

In certain flatworms, sexual reproduction does not require a living intermediate host. Rather, a cocoon containing eggs grows within the flatworm. After having reached a certain size, this cocoon is released by the flatworm and attached onto water weeds or stones. The yolk-containing eggs eventually hatch into new flatworms. (NI) [|1]

Flatworms have the ability to reproduce sexually and asexually because they are Hermaphrodites, meaning they can produce sperm as well as eggs. Most commonly flatworms reproduce asexually but they have the ability to reproduce with other worms. Flatworms can produce millions of eggs in a single day, which are found in the rear section of the worm. The eggs leave the body when ready by breaking off and are then excreted with digested food. The eggs remain in the feces until a host ingests it; eggs do not survive outside of a hosts body for very long so unless the eggs are ingested by another host organism, the eggs with die. (CW)(7) (CW)(7)

Movement:
Platyhelminthes move by using cilia which is located on the ventral epidermis. They move along mucus that they secrete beneath them. Some, however, use and undulating motion to swim through the water. "Movement in some flatworms is controlled by layers of muscle. The development of directional movement has to do with cephalization, which is a tendency for animals to locate their organs near their heads. In some flatworms, the process of cephalization has included the development in the head region of light-sensitive organs called ocelli. Other sense organs found in at least some members of this group (not necessarily on the head) include chemoreceptors, balance receptors (statocysts), and receptors that sense water movement (rheoreceptors)." [MS] [|2]

[The exchange of gases in flatworms, occurs through diffusion. (MP)[|4]] "This attribute of diffusion contributes to the flat shape of the Platyhelminthes. Because all respiration takes place as gases diffuse through the membrane, all parts of the worm must be close to the outer membrane, so a flat shape is necessary to maintain this structure. (JS)[|(3)]"
 * Resiration:**

Most flatworms contain a nervous system, utilizing a processing center called a ganglion, typically located near the anterior end of the flatworm. This primitive brain is connected to longitudinal nerve cords, which run down the length of the flatworm. These nerve cords are connected together via transverse connectives. The flatworm's sensory receptors are seen throughout the system. (TM)(10)
 * Sensing the Environment:**

Flatworms contain **nerve nets**, which are a series of separate, but “connected neurons,” meaning the neurons have the ability to communicate with each other. The nerve nets in flatworms are connected by long nerve cords that connect to the cerebral ganglia in the head region of the flatworm. (CC) ([|11])

Some platyhelminthes have light-sensing "eyes," while some have other other receptors (chemical, balance, and water movement). (14)(SM)

Osmosis:
The excretory system is mainly used to maintain osmotic balance. Flame cells, ciliated cells, allow fluid to excrete through ducts that open up to the environment. One end of the tubule opens through a small pore to the exterior while the other end opens into the interior of the organism, in a spherical structure containing cilia, the flame cells. These flame cells propel the water through the tubules. [[|9]] (LJ) Due to the evolution of osmoregulating structures the organism is now able to survive in fresh water.

Flame cells also regulate the amount of water that is excreted, helping maintain water balance. The pores that the fluid is excreted out of are nephridiopores. Freshwater flatworms' flame cells' main objective is water regulation. (SI) (12)

1. How are flatworms able to move? Include details. (MP) 2. How do flatworms reproduce? How are the eggs dispersed after fertilization? (RG) 3. How do flatworms maintain homeostasis? Describe in detail. (SR) 4. **What are the different ways in which Platyhelminthes are able to eat and digest food? Explain each way. (MF) **
 * Review Questions:**

1. [] 2. [] 3. [] 4. http://library.thinkquest.org/28751/review/animals/3.html 5. [] 6. [] 7. [] 8. [] 9.http://kentsimmons.uwinnipeg.ca/16cm05/16labman05/lb5pg7.htm 10. [] 11.http://faculty.washington.edu/chudler/invert.html 12. http://www.anselm.edu/homepage/bpenney/teaching/BI338/Phyla/Platyhelminthes.html 13. http://www.youtube.com/watch?v=J9EuFuJF9N0(ZS) 14. [] (SM)