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ehd123 ehd123
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11 years ago
One of the first phyla that adopted 3 germ layers.
The endoderm, ectroderm and mesoderm.
The ectoderm gave rise to blood vessels, the outer lining of the gastrovascular cavity, the bladder, dermis, reproductive system etc. 
The ectoderm gave rise to inner lining of digestive tube, and respiratory system.
The mesoderm was responsible for the key development.

The organisms in Phylum platyhelminthes belong to interstitial fauna that live in the water between sand particles. They have bilateral symmetry, where the only plane of division that could cut the body into 2 equal halfs would be the one passing the longitudinal axis  across the midline of the dorsal part. the dorsal and ventral parts are not the same. As organisms started to swim and crawl, bilateral symmerty was imposed by evolution by concentrating neurons at the anterior end of the the organisms to analyze the environment ahead of them. this lead to cephalization of the body and the development of the head. Since the organisms are moving and are more active, smooth muscles appeared and were of 2 types, cicular and longitudinal. longitudinal being on the inside and circular on the outside. When circular muscles contract, the body lengthens and expands, and when circular muscles contract, the body shortens and thickens. Compartmentalization of tissues took place because tissues were wrapped with epithelial tissues, and thus gave rise to smooth muscle organs important for homeostasis. Then the 2 lines of evolution started to appear : protostomes and deuterostomes.
Protostomes are the organisms among the annelids, arthropods, molluscs and insects groups. excluding echinoderms.
Deuterostomes are the organisms that are among the hemichordates, cheatognats and chordates as well as echinoderms.
The difference between the two groups is that in deuterostomes, during embryogenesis, we get a blastula with a pore called blastopore and this becomes the anus.
the protostomes blastopore forms the mouth

Durign mitotics divisions in embryogenesis, the addition of cells dictates the symmetry and pattern of the body of the organism.

in deuterostomes, cleavage was added radially, with new cells added either parallel to the longitudinal axis o zygote or or perpendicular to it.
in protostomes, cleavage was added spirallyy at 45 degree angles to the longitudinal axis f the zygote.

Phylum platyhemithes is a protostome, triploblastic stage, among marine interstitial fauna. They utilize the dissolved organic matter in the sea either by swallowing sea water or diffusion across their skin.
injuries in platyhelmithes organisms could be regenerated by totipotent cells called blastema.

They have internal fertilization and lack a larvae stage unlike many marine organisms, and this means that they have direct development.
They mostly feed on bateria, but may also feed on fungi and microscopic algae. These interstitial fauna are important for recycling water.

If they were in fresh water, osmoregulation is a vital process, and the eggs are placed on vegetation or a substrate. in marine sands, the egg is cemented with the sand. Freshwater eggs are equipped with yolk due to lack of dissolved organic nutrients in water.

They undergo asexual reproduction by parthenogenesis to increase in # quickly, and use in a short time abundant resources which are temporary.and sexually by hermaphroditism, and this is advantageous when the population exists in low densities.
3 classes among the playthelminthes phylum are parasitic: cestoda, trematoda, monogea. Turbelleria is the only free living class among them and possibly the largest class.

They have an excretory system to maintain osmoregulation and excretion under control via protonephridia in flame cells.
The body of flatworms is dorsoventrally flattened, and thin, which rids them of the need to have a circulatry system. Everything moves by diffusion. No respiratory system needed.

An example of the free living turbellarian flatworm is the Planaria species. They have eyespots and auricles to increase surface area, and sensory receptors like statocysts (Gravity receptors) and ocelli ( kight receptors), r mechanoreceptors, or taste receptors and some even have tentacles on their heeads.

Class turberllaria is a filterfeeding class of flatworms, and they feed on bacteria. They are free living, small, and dull in color. They are benthic in freshwater and sea, and few are terrestrial. Their epidermis is multiciliated where the epidermal cells rest on a basal lamina.
among the epithelial cells, they have glands that provide adhesion. they secrete sticky substanes to stick to sand grains like duo-glands found in epidermis. the duo-gland contains 2 types of glands, one secretes adhesive substances to tat the flatworm could adhere to sand grains, and the other secretes a deadhesive when flatworm wants to relocate and move.
also in the epidermis, there are rhabdites that are glands that secrete rod shaped bodies made of proteins substances, such that when proteins exude out of the gland, and contact water,they convert to  a mucus or slime substances for lubrication, movement or gliding on sand.

the body tissues include the outer most epidermis
tbe basal lamina
the smooth muscle layer with circular and longitudinal muscles
thin connective tissue called parenchyma found between the body wall and gut. it is composed of mainly protein fibers called mesenchymal cells and contains among them huge cells used for support called fixed parenchymal cells that are totipotent. epidermal replcement cells that migrate and replace damaged epidermal cells in adult because the epidermis in adult does not undergo mitosis, and they too are totipotent. Neoblasts for wound healing and regeneration of damaged and lost tissue, also totipotent. and chromatophores that are pigment cells that help worm in camouflage. if the pigments were concentrated next to the nucleus, the body appears light in color, whereas if they were dispersed throughout the cytoplasm, they would be darker, and this pigement dispersion is under brain control.

All flatworms are predators. and feed on decaying bodies, bacteria, prtozoans, small invertebrates and crustaceans, worms, anything that falls into sand.
Preys are caught via 3 methods:
the worm wrapping itself around the prey because of adhesive stick glands
immobilizing the prey by sitting on it and pinning it to a substrate and secreting a lot of mucus on it
and entagling the prey with mucus or adhesive substrates

to eat the prey, the mouth which is ventrally located leads to the pharynx which is ciliated and foldered and muscular and withdrawn at rest. this pharynx is called the plicate pharynx and is what goes out to capture prey. when a prey is located, the pharynx extends outside the mouth and is directed towards the prey and digestive enzymes begin external digestion to soften the prey body, and then is retracted to swallow the prey, and direct it towards that branched intestine where extracellular digestion takes place in the lumen further, and then proteases secreted to break prey down to smaller nutrients completely digestion intracellularly sincr food vacuoles exist in interstinal cells.

The nervous system is a bilobed brain with 2 longitudinal nerve cords with many connected side nerve branches. and it is a diffuse brain.

Reproduction in turbellarians is asexual via budding or transverse fission. adult worms secrete adhesives and attach ot a substrate and move pulling its body until it tears into 2 and regenerates the mising part (in freshwater worms )  Neutral Face 
Asexual reproduction takes place in warm months and when the day length is long, whereas sexual takes place during cold months and short days.
in the ovaries, vitellerium is a gland yolk that incorporates yolk into growing oocyte because they are in freshwater and interstitial space.
After mating, sperm are recieved and stored in the seminal bursa. when eggs mature, the sperm migrate and are stored temporarily in semical receptacle. as oocyte is secreted from the ovary, it is fertilized in semical receptacle and lead to the outside of he body.. we get a juvenile directly via direct development.
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B.Sc in Biology
M.Sc Neuroscience
PhD. Candidate in Neuroscience


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