Animal kingdom | Part -2

Structure of Ctenophora, Platyhelminthes, and Aschelminthes

In this article, we will explore the detailed structure of three important phyla: Ctenophora, Platyhelminthes, and Aschelminthes. These animals exhibit different levels of body organization and adaptations suited to their environments.

Structure of Ctenophora

Ctenophores, commonly known as comb jellies, are exclusively marine animals with a transparent, gelatinous body. Their structure is unique and adapted for floating and capturing prey in the water column.

• Body Symmetry: Radially symmetrical with biradial features.
• Body Layers: Diploblastic – composed of ectoderm and endoderm separated by mesoglea.
• Locomotion: Eight rows of ciliary plates (combs) used for movement – the only animals that use cilia for locomotion.
• Feeding Structures: Tentacles equipped with sticky cells called colloblasts to trap prey (no stinging cells).
• Nervous System: Diffused nerve net with statocyst for balance.
• Other Features: Bioluminescent and lack a specialized respiratory or excretory system.

Structure of Platyhelminthes

Platyhelminthes, or flatworms, are a diverse group of animals that may be free-living or parasitic. Their flat body is an adaptation for diffusion-based respiration and nutrient exchange.

• Body Symmetry: Bilaterally symmetrical and dorsoventrally flattened.
• Germ Layers: Triploblastic – having ectoderm, mesoderm, and endoderm.
• Body Cavity: Acoelomate – no body cavity present.
• Digestive System: Incomplete in most – single opening for ingestion and egestion; some lack it entirely.
• Excretory System: Flame cells (protonephridia) help in osmoregulation and excretion.
• Nervous System: Ladder-like structure with paired ganglia and longitudinal nerve cords.
• Reproductive System: Mostly hermaphroditic with high regenerative capacity.

Structure of Aschelminthes

Aschelminthes, also known as roundworms or nematodes, exhibit a more advanced body plan than flatworms. They are cylindrical, unsegmented, and found in various habitats.

• Body Symmetry: Bilateral symmetry with a smooth, elongated body.
• Germ Layers: Triploblastic with well-developed tissue organization.
• Body Cavity: Pseudocoelomate – a false body cavity not lined by mesoderm.
• Digestive System: Complete with a distinct mouth and anus (tube-within-a-tube structure).
• Body Covering: Tough, elastic cuticle that resists digestion by host enzymes (in parasitic forms).
• Excretory System: Renette cells or tubular excretory systems.
• Nervous System: Simple nerve ring with dorsal and ventral nerve cords.
• Reproductive System: Dioecious (separate sexes), often with internal fertilization.

Summary Table: Comparative Structural Features

Each of these phyla — Ctenophora, Platyhelminthes, and Aschelminthes — demonstrates a different level of structural complexity.