Biologie des Spiraliens et Plathelminthes

SPIRALIENS

• A large group of bilaterian animals, encompassing Plathelminthes, Annelids, and Molluscs.
• Characterized by spiral cleavage during embryonic development.
• Demonstrate a shared evolutionary history.

PLATHELMINTHES

• General Characteristics: Flattened, unsegmented bodies; lack a coelom; a simple digestive system (often without an anus); acoelomate.
• Body Morphology: Dorso-ventrally flattened; lacking body segmentation; variable size, ranging from millimeters to a few centimeters.
• Habitat: Free-living forms inhabit marine, freshwater, and damp terrestrial environments. Parasitic forms live in a variety of hosts. Majority of species are parasitic (85%), living inside one or more hosts.
• Simple Characteristics: Lack a circulatory and respiratory systems, with a simple digestive cavity and a high surface area to volume ratio, which facilitates nutrient uptake and gas exchange.
• Complex Characteristics: Hermaphroditic (most species); complex reproductive systems; and adaptations to parasitic lifestyles (e.g., reducing sensory organs, complex holdfasts, and manipulation of the host's immune system).
• Regeneration: Remarkable regenerative abilities, particularly in planarians.

ANNELIDS

• General Characteristics: Segmented bodies; a coelom; a complete digestive system; a closed circulatory system.
• Body Morphology: Segmented bodies; most annelids are elongated with a series of repeating segments.
• Habitat: Inhabit a diverse range of aquatic and terrestrial environments.
• Coelom: The coelom is a body cavity lined with mesoderm, facilitating internal organ development and space for the growth of internal organs.
• Circulatory system: A closed circulatory system, with blood contained within vessels, allowing for efficient circulation of nutrients and oxygen throughout the body.
• Nervous system: A well-developed central nervous system.
• Key groups: Errantia (mobile species); Sedentaria (mostly sessile species); Clitellata (a group that includes earthworms).

MOLLUSKS

• General Characteristics: Soft-bodied animals; typically with a mantle, a muscular foot, and a visceral mass; often with a shell made of calcium carbonate.
• Body Morphology: A soft body typically covered in a mantle; usually a muscular foot used for locomotion; possessing a visceral mass containing internal organs.
• Habitat: Predominantly marine habitats but also inhabit freshwater and terrestrial environments.
• Diversity: Show wide variations in form and function, with many different species occupying various ecological niches.
• Key Groups: Conchifera (e.g., bivalves); Gastropods (e.g., snails); Cephalopods (e.g., octopus).
• Evolutionary features: The evolution of a complex nervous system and sensory organs contributes to their success as a group.