Animals- Phylum Platyhelminthes

Platyhelminthes = flatworms

• Bilateral symmetry and dorsoventrally flattened bodies.

• Exhibit cephalization (head).

• Acoelomate: lack a body cavity, possess 3 cell layers (3 germ layers).

• Gastrovascular cavity acts as a stomach.

• Have protonephridia (primitive kidneys).

• Hermaphroditic: both male and female reproductive organs.

• Mostly aquatic.

• Examples: Turbellaria (Planaria), Trematoda (Flukes), and Cestoda (Tapeworms).

Structure/Anatomy

• Planarian Anatomy:

  • Digestive system includes intestine, pharynx, pharyngeal cavity and opening, and mouth.

  • Excretory system includes flame cells, excretory pores, and excretory tubules.

• Tapeworm Anatomy (Taenia):

  • Scolex with rostellum (hooks) and suckers for attachment.

  • Proglottids: Mature and Gravid.

  • Includes structures such as ventral and dorsal excretory canals, genital pore, cirrus sac, testes, ovaries, and uterus.

Feeding

• Free-living flatworms: Carnivores or scavengers.

  • Use a gastrovascular cavity connected to a mouth via an extendible, muscular pharynx.

  • Food is absorbed by epithelial cells and digested intracellularly.

• Parasitic flatworms: Feed on blood, tissue fluids, or cell fragments within a host.

  • Often lack a complete digestive system; absorb already broken-down nutrients.

Respiration, Circulation, and Excretion

• Thin bodies allow diffusion between tissue layers for respiration and circulation.

• Excretion: Flame cells remove excess water, functioning as primitive kidneys.

Response to Environment (Nervous System)

• Free-living flatworms: Primitive organisms to display cephalization.

  • Have brain-like ganglia (nerve cell groups).

  • Specialized eyespots detect light.

Reproduction

• Free-living flatworms: Asexual fragmentation; sexual reproduction via gamete transfer.

  • All individuals are hermaphrodites.

• Parasitic flatworms: Complex life cycles involving secondary hosts for sexual reproduction.