Acoelomates

First, to actively seek food, shelter, and mates

Bilateral Symmetry

Three Phyla of Acoelomates:

Platyhelminthes - flatworms

Nemertea - ribbon worms

Gnathostomulida - jaw worms

General Characteristics:

- Three distinct germ layers:

- No Coelom, region between ectoderm and endoderm is filled with mesenchyme or parenchyma (mesoderm)

- do have an internal cavity or digestive cavity (gastrovascular)

- Organ-system level of organization

- excretory system, circulatory system in some

Phylum Platyhelminthes - 4 classes: we discuss 3

1. Class Turbellaria

free living ciliated epidermis

Locomotion -

Digestive System - Mouth, pharynx, and intestine (lined with columnar epithelium); branching system allows food to be distributed to all parts of body

Extracellular digestion by proteolytic enzymes AND intracellular by phagocytic cells

Incomplete Digestive System OR One Way Alimentary Canal

Osmoregulation -

Excretion and Respiration -

Nervous System -

ladder like pattern

Sense Organs - Ocelli, Auricles, Statocysts

Reproduction and Regeneration

asexual reproduction or fission. Easy to repopulate when numbers are low

sexual reproduction. hermaphroditic (monoecious) - but cross fertilize.

2. Class Trematoda parasitic flukes; endoparasites of vertebrates

Syncytial tegument

Multinucleate cells

Tegument better word than epidermis

Parasitic Adaptations:

Increased reproductive activity

Adhesive organs -

Complex life cycles:

1. egg --> from human in feces ----> needs water

2. egg hatches to miracidium larvae that can live for several weeks in water.

3. miracidium penetrates tissues of snail and becomes sporocyst

4. sporocyst reproduces asexually to yield either more sporocysts or rediae (elongated with alimentary canal, nervous and excretory system.

5. rediae pass to liver of snail

6. rediae give rise to more rediae or produce tadpole like cercariae

7. cercariae emerge from snail to water and penetrate second intermediate host, a fish, or encyst in vegetation (juvenile flukes)

8. bore into fish muscle as cysts

9. mammals eat fish and flukes migrate to bile duct and become adults that lay eggs.

Digestive system - pharynx, esophagus and an intestine that is unbranched.

Osmoregulation - similar to turbellarians with flame cells but a single median bladder with single excretory pore.

Other flukes:

Schistosoma a blood fluke - Schistomiasis

Paragonimus a lung fluke

3. Class Cestoda tapeworms

protoglottids

New protoglottids added at anterior end so ones at posterior end of animal are older and more mature.

no digestive system sucker with spines - scolex

Pork tapeworm - adults live in intestine of humans; juveniles in muscles of pigs. Embryos can migrate to eye or brain. Cysticercosis

Phylum Nemertea -- ribbon worms

similar to Platyhelminthes in that they have:

ciliated epidermis, protonephridia with flame cells. BUT are dioecious

Unique characteristics: eversible proboscis

Anus = complete digestive system

Simplest animals to have a Blood vascular system - no heart but some enclosed blood vessels.

Phylum Gnathostomulida

Jaw worms with no circulatory system and no anus