Platyhelminthes
Check out this website for brief review of planarians/platyhelminthes: http://snowbio.wikispaces.com/Planaria+(Platyhelminthes)
image from: http://cnx.org/content/m48096/latest/
General Characteristics
- Usually dorsoventrally flattened.
- Triploblastic.
- Acoelomate.
- Bilaterally symmetrical
- Unsegmented worms.
- Incomplete gut; however gut absent in Cestoidea.
- Cephalized, with an anterior cerebral ganglion, with longitudinal nerve cords.
- Monoecious; complex reproductive systems
- Gastrovascular Cavity
image from: http://rmhdissectionlog.blogspot.com/2010_12_01_archive.html
Cells, Structures
- Protonephridia excretory/osmoregulatory structures.
- Pair of anterior ganglia in the nervous system; Longitudinal nerve cords connected by transverse nerves and located in the mesenchyme.
Reproduction
- Tubellarians reproduce asexually by transverse fission.
- The two or more animals that result from fission are zooids, regenerate missing parts after separating from each other.
- Monecious.
- reproductive systems arise from mesodermal tissues in the parenchyma.
- Sperm ducts lead to a seminal vesicle and a protrusible penis; which projects into a genital chamber.
Classes
Turbellaria |
Characteristics: brightly colored,beat cilia in water to move
Body Wall: Connective tissue and muscle, endodermis derived from gastrodermis Digesting: one opening, carnivorous, scavenger, herbivore, algae feeder Digest: extracellular, phagocytic Rhabdites: outer protective sheath Adhesive glands: chemically attaches Releaser glands: chemically breaks down adhesive |
Gas Exchange: protoephridia is a network of tubes
Flame Cells: have numerous cilia projected into the lumen of the tubule. Openings perforate the tubule wall surround the flame cell. Beating of the cilia drives fluid down the tubule, creating a negative pressure in the tubule resulting in the fluid from the surrounding tissue being sucked through the openings into the tubule. Nephridiopore is a minute opening that leads merging tubules to expel fluids. |
Locomotion: Glide as bottom dwellers using both cilia and muscular undulations but primarily muscular.
As they move they lay down a sheet of mucus which aids in adhesions and helps cilia gain traction. The densely ciliated ventral (bottom) surface and flat bodies enhance locomotion. Nervous System and Sensory Organs: Most primitive is the nerve plexus in the order Acoela which resembles the cnidaria's nerve net. A Statocyst is a receptor for pressure detecting the turbellarian's body position in reference to the direction of gravity. |
Some have a more centralized nerve net with cerebral ganglia.
Most turbellarians have a subepidermal nerve net and several pairs of long nerve cords. Lateral braches called commissure connect the nerve cords . They have neurons allowing for sense, motor, and communication between the sensory and motor neurons. nervous tissue is concentrated into a pair of cerebral ganglia (primitive brain). Auricles projected sensory lobes from the side of the head use chemoreceptors aiding in locating food. Ocelli are eyespots to sense light |
Turbellarian Reproduction and Development
image from: http://snowbio.wikispaces.com/Planaria+(Platyhelminthes)
Reproduce asexually by transverse fission resulting in two or more zooids which regenerate missing parts after separation. Reproductive systems arise from the mesodermal tissues in the parenchyma. Sperm ducts (vas derens) lead to a seminal vesicle (sperm storage) and a protrusible penis. Female system has one to may pairs of ovaries which include oviducts that opens through the genital pore.
Reproduce usually from reciprocal sperm exchange. This ensures genetic diversity via cross-fertilization.
Eggs are laid with or without a gel like mass which is then enclosed in a cocoon which attaches to the stubstrate.
Develop usually straight from embryo into an adult. Few turbellarians have a free-swimming stage called a Muller's larva which have ciliated extensions for feeding and movement. The larva eventually settles unto a substrate developing into a young tuberellian.
Reproduce usually from reciprocal sperm exchange. This ensures genetic diversity via cross-fertilization.
Eggs are laid with or without a gel like mass which is then enclosed in a cocoon which attaches to the stubstrate.
Develop usually straight from embryo into an adult. Few turbellarians have a free-swimming stage called a Muller's larva which have ciliated extensions for feeding and movement. The larva eventually settles unto a substrate developing into a young tuberellian.
More Classes
Monogenea
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Trematoda
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Trematoda Subclasses
Aspidogastrea
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Digrenea
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Chinese liver fluke
Fasciloa hepatica
Schistosomes
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