Phylum Platyhelminthes

General Characteristics

1) Triploblastic (applies all three phyla of acoelomates).

2) Bilateral symmetry (applies all three).

3) Body is flattened dorsoventrally (compressed dorsoventrally).

4) Ectoderm is very specialized w/ distinct epidermis in all three phyla. *Note: specialized "flatworm" characteristics associated externally w/ epidermis.

a) some spp. epidermis is syncitial

1) Having one/few epidermal cells having many nucleus. Usually elaborating cilia in many spp. Aid in locomotion and protection.

b) rhabdites - specialized epidermal cells and extracellular structures of many flukes, which aid in locomotion.

c) Syncytial tegument - extraepidermal layer of one/few multinucleated cell(s) w/ protective covering. In many flukes.

d) Same cellular extraepidermal structures and epidermis. One nucleus/cell. Multicellular arrangement of epidermis and sometimes ciliated structures.

5) Muscular development and arrangement into simple body wall muscles used mainly for locomotion.

6) No body cavity (acoelomate).

7) Digestive tract is incomplete:

a) No mouth (oral opening directly into pharynx).

b) No true stomach structure (pharynx opens into a complex "intestinal" structure). In some spp. there is no intestinal structure but rather GVC-like cavity. No contact w/ mesoderm.

c) In tapeworms, there is no digestive tract; absorption of nutrients is direct through skin (epidermis) and/or through oral opening. Direct absorption of soluble nutrients by cells and tissues.

8) Simple arrangement of nerve cells and sensory organs (EX: eyespots of planaria).

9) Sense organs

10) Excretion - mostly passive diffusion of soluble wastes; some specialized cells and organs protonephridia cells (("primitive excretory cells") (flame cells)) make up simple excretory organs

11) Respiration (gas exchange by simple diffusion) no circulatory system therefore intercellular diffusion and transport from cell to cell.

12) Reproduction

a) Sexual : monoecious spp.

b) Asexual : regeneration and fission

13) Range from free-living planaria to parasitic flukes and tapeworms.



Class Turbellaria - common name planaria - scientific name Dugesia sp.

1) Free-living

2) Mostly freshwater spp.


Class Monogenea

1) Mostly parasites of fish in skin/gills.

2) Free-living larval stages

3) Ciliated, mobile larvae

4) Some parasitic flukes in class


Class Trematoda - genus Fasciola spp. - Clonorchis spp.

1) Major parasitic flukes

2) Includes blood flukes and liver flukes.


Class Cestoda - tapeworms - Taenia spp. - pork tapeworm

1) All spp. are parasitic, which lack any type of digestive tract.



Focus on Class Turbellaria

1) Range from roughly 5 mm -> 50 cm

2) Usually covered w/ ciliated epidermis - cilia used mostly for locomotion; cilia sometimes used to create water currents to draw water and food materials near pharynx and into digestive tract.


Form and Function of planaria


a) Ciliated epidermis - specialized rhabdites, rod shaped wells of epidermis. Rhabdites water- filled; Can discharge a release of water and a protective mucous sheath around body of organism (mucous layer is composed of complex polysaccharides). Carbohydrate polymers, simple and complex saccharides (sugars) monosaccharides monomeric simple sugars. Glucose (C6H1206 or CH20) general formula for carbs.

1) glucose - used for two types of metabolic processes

a) Catabolism : glucose breakdown (enzymatically) to yield energy in form of ATP.

b) Anabolism: biosynthesis; "building" complex organic molecules (enzymatically) from simple starting materials such as glucose, e.g. glucose polymerize, complex saccharides such as:

1) starch : energy storage form of example.

2) cellulose : structural polysaccharide of glucose

3) glycogen : energy storage form of glucose

4) chitin : structural form of glucosamine

5) mucosal polysaccharide : complex forms of glucose and many other carbohydrates to make mucous layers and/or structures.

2) Polysaccharide materials

a) protection

b) prevents desiccation (water loss under adverse conditions)

c) facilitate locomotion

d) capture food particles


Complex arrangement of body muscles attached to endodermal, mesodermal, and ectodermal material, allowing contortion of body in several different planes relative the longitudinal, central axis thus allow facilitation of locomotion. "Swimming" mobility - collective use of cilia, muscles, mucous layer for locomotion. Muscle fibers arranged circularly, laterally, and longitudinally.


Nutrition and Digestion : digestive tract is incomplete

1) "mouth" : oral opening (very simple and unspecialized) - simply an opening through which pharynx (pharyngeal sheath) can be extended.

2) Pharynx : pharyngeal sheath. Opens externally toward posterior region of organism and can be extended through oral opening; connects internally, directly to intestines.

3) Intestines : major digestive and adsorptive region of digestive tract; very highly branched and extends throughout bulk of interior of body. Aids in intercellular transport (directly) of nutrients to other cells and tissues of body (active transport and passive diffusion of partially metabolized structure). The inside (lumen) of intestines is highly folded (convoluted) to provide greater surface area for more efficient adsorption of nutrients w/in intestines.

4) Digestion of food particles is extracellular by digestive enzymes secreted by specialized cells of intestinal lumen.


Excretion and Osmoregulation

Removal insoluble wastes (for example cellulose) no external structure for removal of wastes.

Removal soluble wastes: ionic forms of materials, toxic waste products

Protonephridis- simple, primitive organ-like structures which remove ions and control osmotic pressure of cells of organism and constitute tissues (similar to role of contractile vacuole of protozoa). Unicellular "animals".