There are some features that all flatworms share.They have cephalization, including a distinct brain, and are bilaterally symmetrical. This means that we can tell the difference between anterior, posterior, dorsal, and ventral sides of flatworms. Platyhelmenths have no internal body cavity, meaning that they are acoelomates, but they do have 3 cell layers (endoderm, mesoderm, and ectoderm), just like humans. Below, you can see the body plan of a typical flat worm.
Reproduction, feeding and sensory perception all very among the different classes of flatworms, but all classes rely on diffusion to get oxygen to their body cells.
4 classes of flatworms:
1. Turbellaria - like the planaria! There are around 4,500 species known in marine, fresh water and moist terrestrial habitats. Nearly all Turbellarids are hermaphrodites and reproduce sexually, though they can also reproduce asexually by fragmentation (similar to regeneration). Sexual reproduction in this class can be a little unusual, so much so that they made it onto "NatGeo Wild's" series World's Weirdest for a behavior called "penis fencing." If you're curious, here is the clip! These flatworms are the only class with specific sensory organs, and they have eyespots, which respond to light.
2. Tremetoda -like flukes. There are as many as 24,000 known trematode species, and all are parasites which include 2 hosts in their lifecycle. Most species use snails as intermediate host before infecting fish and even humans. Depending on the species, flukes can infect the skin, blood, or organs like the lungs and liver. Freshwater is a prime habitat for infection. Occasionally, lakes are closed for swimming due to presence of the flukes in the water. In the USA, nearly all species will only infect the skin, causing a rash, because humans are not the primary host, but that is not the case when traveling to places where water is used for washing purposes. Check it out here from"Animal Planet's" Monsters Inside Me. Below is the lifecycle of a type of trematode knowns as schistosoma. The cycle is complicated but notice the key points:
A. Sexual Reproduction occurs in the human host
B. Eggs are excreted in human feces and urine and end up in water
C. Once in water, the eggs hatch and the young (miracidia) infect water snails.
D. The miracidia grow within the snail and enter the water in another form, the cercariae.
E. Humans in the water can become infected when the cercariae enter the skin.
F. Once in the human, the cercariae become adult worms, which mate and lay more eggs. --> the cycle continues
3. Cestoda- the tapeworms! YUMMMM! This is another class of platyhelminths where all of the species are parasites. The tapeworm gains at the host's expense. An infected host will consume food, digest it, and then the tapeworm within the hose absorbs the nutrition from the food. Again, a class of Platyhelmenthes are so interesting that they made another prime time feature on "Animal Planet's" Monsters Inside Me. Check it out here!
Rumor has it that people once intentionally consumed tapeworms to lose weight because the worm would block in absorption of calories. Talk about an "organic" weight loss plan!? But look at this creature - would you really want that inside of you?! They have hooks on their head, which they bury into stomach or intestinal lining, and they grow... and they grow... and they grow... and the record is: almost 29 Feet! YUCK.
What is particularly scary is that there are a wide variety of ways to get infected, which depends on the species, and not all tape worms go for our guts. Some tapeworms live inside of fleas, which can jump off of your dogs and into your mouth, eventually infecting you. They can also enter the body through contaminated food (especially fish and pork). Certain species can even directly enter the skin (usually on cuts and sores). Depending on the species, the worms can live in the guts, skin, or even the brain. And, they can reproduce fast. Each body segment, called a proglottid has male and female parts, which can self fertilize, and when the proglottids break off, they can form all new worms! Ok, Nightmare? Here is the life cycle:
4. Monogenea are a class of small flatworm which at ectoparasites (parasites that live on the outside) of marine fish fins and gills. Compared to the other three classes, they are very simple animals with no intermediate hosts, and only one species is know to infect an animal other than fish. That species actually infects the eye of hippopotamus!
No comments:
Post a Comment