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