Acoelomate Bilateral Animals
Acoelomate Bilateral Acoelomate Bilateral AnimalsAnimals
• Consist of phyla:– Phylum Platyhelminthes
– Phylum Nemertea
– And others
Acoelomate Bilateral Acoelomate Bilateral AnimalsAnimals
Reproductive and osmoregulatory systems
1. Simplest organisms to have bilateral symmetry
2. Triploblastic
3. Lack a coelom
4. Organ-system level of organization
5. Cephalization
6. Elongated, without appendages
Bilateral SymmetryBilateral Symmetry
• Divided along sagittal plane into two mirror images– sagittal= divides bilateral organisms into
right and left halves
ReviewReview• Anterior= head end
• Posterior= tail end
• Dorsal= back side
• Ventral= belly side
Bilateral animalsBilateral animals
• Bilateral symmetry = important evolutionary advancement– Important for active, directed movement
•Anterior, posterior ends
– One side of body kept up (dorsal) vs. down (ventral)
Directed movement evolved with anterior sense organs cephalization
Cephalization– specialization of sense organs in head
end of animals
• Acoelomates lack a true body cavity– Solid body
– no cavity b/w the digestive tract and outer body wall
This is a round wormDifferent Phylum
AcoelomatePhylum PlatyhelminthsOr not shown hereNemerterean
Acoelomates are Acoelomates are triploblastictriploblastic
• Triploblastic (3 germ layers)– Germ layer= layers in
embryo that form the various tissues and organs of an animal body
3 germ layers
• Ectoderm– Outermost germ layer
– Gives rise to outer covering of animal ie. epidermis
• Endoderm– Innermost germ layer
– Gives rise to inner lining of gut tract
• Mesoderm– Middle germ layer
– b/w ectoderm and endoderm
– Gives rise to various tissues/organs (ie. muscles)
Acoelomate animals have Acoelomate animals have an organ-system level of an organ-system level of
organizationorganization
Acoelomate animals have an Acoelomate animals have an organ-system level of organizationorgan-system level of organization
Digestive tract and nervous system
• Organ-system– Different organs
operate together (ie. excretory system, nervous system)
– mesodermal tissue gives rise to parenchyma
PolycladPolyclad
http://www.rzuser.uni-heidelberg.de/~bu6/
• From Red Sea
Phylum Phylum PlatyhelminthesPlatyhelminthes
Flatworms Free living
Parasitic
• From Atlantic ocean
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Phylum PlatyhelminthesPhylum Platyhelminthes
• Flattened dorsoventrally– flatworms
• 34,000 species
• Gastrovascular cavity (if present) has only one opening (mouth = anus)
• Mostly monoecious
Phylum PlatyhelminthesPhylum Platyhelminthes
• First phylum that has an Organ systems present – derived mesodermally
(parenchyma):•Muscular system
•Digestive system (incomplete; gastrovascular type) (absent in some)
•Nervous system
•Excretory system (absent in some)
•Reproductive system
Phylum PlatyhelminthesPhylum Platyhelminthes
• Organ systems absent:– Circulatory
– Respiratory
Rely on diffusion
Phylum Platyhelminthes Phylum Platyhelminthes (cont’d)(cont’d)
Hymenolepsis- rat tapeworm
• Divided into 4 classes:– Class
Turbellaria (mostly free-living flatworms)
– Class Trematoda (parasitic flukes)
– Class Monogenea (ectoparasitic flukes)
– Class Cestoda (tapeworms)
Class TurbellariaClass Turbellaria
• Mostly free-living flatworms
• Marine (mostly) or freshwater bottom-dwellers
• Predators and scavengers
• First group of bilateral symmetrical animals
Planarian genus Dugesia
• The best-known turbellarians, commonly called planarians
– Have light-sensitive eyespots and centralized nerve nets
Pharynx. The mouth is at thetip of a muscular pharynx thatextends from the animal’sventral side. Digestive juicesare spilled onto prey, and thepharynx sucks small pieces offood into the gastrovascularcavity, where digestion continues.
Digestion is completed withinthe cells lining the gastro-vascular cavity, which hasthree branches, each withfine subbranches that pro-vide an extensive surface area.
Undigested wastesare egestedthrough the mouth.
Ganglia. Located at the anterior endof the worm, near the main sourcesof sensory input, is a pair of ganglia,dense clusters of nerve cells.
Ventral nerve cords. Fromthe ganglia, a pair ofventral nerve cords runsthe length of the body.
Gastrovascularcavity
Eyespots
Figure 33.10
Class Turbellaria (cont’d)Class Turbellaria (cont’d)
• Move by muscles, ciliated epidermis
/gastrovascular cavity
Class Turbellaria (cont’d)Class Turbellaria (cont’d)
• Freshwater turbellarians adapted osmoregulatory structures– Protonephridia
•protos= first
•nephros= kidney
•network of fine tubules running down sides of organism
Class Turbellaria (cont’d)Class Turbellaria (cont’d)
– Flame cells= branch from tubules•Ciliary projections
drive fluid down tubule
– Tubules open to outside= nephridiopore
Class Turbellaria (cont’d)Class Turbellaria (cont’d)
• nervous system with nerve ganglion– ganglion- aggregation of nervous tissue
• Cephalization- cerebral ganglion= primitive brain
Class Turbellaria (cont’d)Class Turbellaria (cont’d)
• Ocelli= light-sensitive eyespots
Turbellarian ReproductionTurbellarian Reproduction
Reproductive and osmoregulatory systems
• Asexual (fission)– transverse
• Sexual– Monoecious
(mostly)
– Cross-fertilization
Other 3 classes:– Class Trematoda
– Class Monogenea
– Class Cestoda
• All parasitic• lack cilia• Have unusual body
covering: tegument
• Outer zone of tegument (glycocalyx)
• consists of proteins and carbohydrates
• aids in transport of nutrients, waste, gases
• Protection against host defenses
Class Trematoda Class Trematoda
• Parasitic flukes
• Endoparasites– Hooks, suckers,
increased reproductive capacity
• 1mm-6cm long
• Complex life cycle:
1. Definitive host (primary/final host) – where
parasite matures and reproduces (sexually) (eggs released)
– vertebrate
2. Intermediate host – Mollusc (ie.
snail)
– Hosts in which larval stages develop and undergo asexual reproduction
– Results in an increase in the number of the individuals
• Trematodes that parasitize humans– Spend part of their lives in snail
hosts
These larvae penetratethe skin and blood vessels of humans working in irrigated fields contaminated with infected human feces.
Asexual reproduction within a snail results in another type of motilelarva, which escapes from the snail host.
Blood flukes reproduce sexually in the human host. The fertilized eggs exit the host in feces.
The eggs develop in water into ciliated larvae. These larvaeinfect snails, the intermediate hosts.
Snail host
1 mm
Female
Male
5
2
3
4
Figure 33.11
Mature flukes live in the blood vessels of the human intestine. A female fluke fits into a groove running the length of the larger male’s body, as shown in the light micrograph at right.
1
Chinese Liver FlukeChinese Liver Fluke• Infects 30 million
people in eastern Asia
• Lives in ducts of liver– Eats epithelial
tissue, blood
• Definitive host:– Humans, dogs,
cats
• 2 intermediate hosts: – snail – fish
Class MonogeneaClass Monogenea
• Parasitic flukes
• Mostly ectoparasites
• Single host, mostly fish
Class CestodaClass Cestoda
• Tapeworms
• Endoparasites
• Vertebrate host– Live in
digestive tract
• 1 mm- 25m long
(EWWWW!!)Hymenolepsis- rat tapeworm
TapewormTapeworm
• Tapeworms– Are also parasitic and lack a
digestive system
Proglottids withreproductive structures
200 µm
HooksSucker
Scolex
Figure 33.12
Class CestodaClass Cestoda• Highly
specialized
• Lack mouth, digestive tract– Absorb
nutrients across body wall
• Hooks and suckers– “head”=
scolex
Hymenolepsis- rat tapeworm
• Adult tapeworms consist of long series of repeating units= proglottids
• Chain of proglottids= strobila
• Tapeworms are monoecious (mostly)– Mostly cross-fertilization
• No specialized sense organs
scolex
• Cestodes depend on host digestion– Small
molecules in host intestine, liver
• Definitive host= human
• Intermediate host= cattle
Beef Beef TapewormTapeworm
Phylum NemerteaPhylum Nemertea• Triplobastic,
acoelamate
• bilateral symmetry
• Unsegmented
• Ciliated epidermis
• Closed circulatory
• usually <20cm
• Marine mud, sand
• Elongate, flattened worms
Phylum Nemertea Phylum Nemertea (cont’d)(cont’d)
• Unlike the platyhelminthes, Complete digestive tract, with anus– One-way
– More efficient; allows larger growth
Phylum Nemertea (cont’d)Phylum Nemertea (cont’d)
• Cerebral ganglion, longitudinal nerve cords
• Long proboscis used in carnivorous species
• Two lateral blood vessels yet no heart
• Dioecious– “two” “house”
– Male and female organs in separate individuals