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Systematic Adaptations of Phylum Platyhelminthes
The Flatworms
Organ level of organization◦ Well developed, specialized anatomical structures
Incomplete gastrulation ◦ Gastrovascular cavity with one opening
Triploblastic◦ True mesodermal tissue filling space between
endoderm and ectoderm (note “parenchyma”) Bilateral Symmetry
◦ 2 symmetrical sides about longitudinal axis Cephalization
◦ Concentration of sensory apparatus at anterior end
Derived Characteristics
Acoelomate Parenchyma derived from mesodermal
tissue Elongated body No appendages
Body Form & Development
Simple integument Ciliated in free living flatworms, but lacking
cilia in parasitic forms Glandular tissue associated with skin
◦ Mucous glands◦ “Dual gland” for adhesion and release
Integumentary
Hydrostatic Skeleton Parenchymal tissue holds a constant
(“static”) volume of water (“hydro”) Compression/contraction in one direction
must necessarily result in expansion/elongation in a different direction
Well developed muscles are present in the body wall◦ Longitudinal◦ Circular◦ Oblique
Skeletal & Muscular
Predict the result from the contraction of:◦ Longitudinal muscle (2)◦ Circular muscle (1)◦ Oblique muscle (3)
Muscular arrangement
https://www.youtube.com/watch?v=zCH37KI_R_E
https://www.youtube.com/watch?v=HekwSO6dBG0
https://www.youtube.com/watch?v=OCOYrHAkjVE
Watch these flatworms move
Centralized nervous system◦ Paired solid, ventral
nerve cords◦ “Ladder” type nervous
organization – Transverse nerves connecting longitudinal nerve cords
Concentration of nerve tissue forming cephalic ganglia (simple brain)
Nervous & Sensory
Cephalization◦ Concentration of
sense organs in anterior body region
Sense organs◦ Photoreceptors (note
“ocelli”)◦ Chemoreceptors
(note “auricles”)
Nervous & Sensory
The development of the digestive cavity is incomplete, forming a gastrovascular cavity with a single opening
Digestion is largely extracellular, although some flatworms complete the process in food vacuoles (intracellular)
The class turbellaria are free living and carnivorous, and have well developed gastrovascular cavities
The other classes of flatworms are parasitic, dependent upon the host for digestion
Digestion
Mouth located ventrally Free living turbellaria have an extensible pharynx The highly branched gastrovascular cavity brings
digested food in close proximity to all body tissues As the gastrovascular cavity has only 1 opening,
wastes are excreted through the pharynx or retained
Digestion
https://www.youtube.com/watch?v=9QJF93WlNew
https://www.youtube.com/watch?v=eUmOJ6fWN9w&feature=endscreen&NR=1
https://www.youtube.com/watch?v=Ich4Y_K6GuM
Watch these flatworms feeding
The gastrovascular cavity in parasitic forms is diminished (as in this photo) or absent (as in the tapeworms)
When present, the gastrovascular cavity is generally Y shaped
Digestion
Osmoregulation is the removal of excess water
Excretion is the removal of metabolic waste (mostly nitrogen compounds like ammonia or urea that accumulate from metabolism of protein) and excess dissolved materials (salts)
The flame cells are concerned with osmoregulation
Metabolic wastes are generally removed by simple diffusion
Osmoregulation vs. Excretion?
Excretion & Osmoregulation
“Protonephridia” – primitive excretory tubules driven by Flame Cells.
The “flame” consists of several flagellae that drive fluids through tubules that lead to collecting tubules
Excretion & Osmoregulation
There is considerable variation in the structure of osmoregulation systems in the flatworm classes, from simple excretory pores to a bladder
Due to the simplicity of the flatworm body form, many are capable of reproducing asexually. In low density populations, asexual reproduction (by budding or fission) may be predominant
Flatworms are monoecious (one reproductive form – AKA hermaphroditic) but generally are unable to self-fertilize
Parasitic forms have complex life cycles, often involving one or more intermediate hosts.
Reproduction
This flatworm is free living. Its muscular and digestive systems, while not shown above, are well developed
Note the presence of both male and female gonads (Monoecious)
Flatworm reproductive organs
This liver fluke is parasitic. It depend on its host for locomotion and digestion, allowing it to devote more of its energy and anatomy to reproduction
Note that is also is monoecious. The testes are more pink, and located posteriorly
The ovary is darker and more centrally located
Fluke reproductive organs
Tapeworms are parasitic. Their digestive system is absent. Almost their entire anatomy is devoted to reproduction
They have a segmented body, with both testes and ovaries located in each segment
Tapeworm reproductive organs
Life cycle of the Liver fluke
https://www.youtube.com/watch?v=r7okJ6wHYLA
https://www.youtube.com/watch?v=EEBbtwGqPEs
https://www.youtube.com/watch?v=9fmx24FbZoM
Trematode Life Cycle
The Phylum platyhelminthes is divided into 4 classes
◦ Class Turbellaria – The free living flatworms
◦ Class Trematoda – The digenetic flukes
Digenetic = 2 or more hosts
◦ Class Monogenea – The monogenetic flukes
Monogenetic = 1 host
◦ Class Cestoda – The tapeworms
◦ https://www.youtube.com/watch?v=wZBPqITnyME
Classes of Platyhelminthes
The only class of free living flatworms Ciliated epidermis with mucous secreting
rhabdites Acoelomate Extensible pharynx Includes the planaria
Class Turbellaria
Parasitic flukes Syncytial tegument (multinucleate skin) without
cilia Y shaped gastrovascular cavity Oral sucker for obtaining food and ventral sucker
for attachment No hooks for attachment to host Digenetic – 2 or more hosts
◦ Intermediate host – usually an invertebrate, frequently a mollusc
◦ Definitive host – a vertebrate Includes fasciola, schistosoma and clonorchis
Class Trematoda
Parasitic flukes Syncytial tegument (multinucleate skin)
without cilia Free living, ciliated larvae Y shaped gastrovascular cavity A single oral sucker A series of posterior hooks (opisthaptor) for
attachment to the host Monogenetic – parasitic on a single host,
usually a fish, amphibian or reptile
Class Monogenea
Tapeworms, parasitic to the digestive tracts of vertebrates Syncytial tegument without cilia The tegument is covered with fingerlike extensions to
increase the surface area for food absorption Gastrovascular cavity is absent Anterior structure (scolex) with hooks/suckers for
attachment to the host Body is formed into segments called proglottids. Each
proglottid has testes and ovaries. Self-fertilization is possible
Gravid proglottids from feces of infected animals may be consumed by a new host, or may be spread by consuming larvae in the flesh of an infected animal
Class Cestoda
https://www.youtube.com/watch?v=Q5BuXmDIJxs
https://www.youtube.com/watch?v=9qyEfeg-334
Tapeworms
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