Biology: Levels of Organisation in Animals

Classification of Animals

• Animals are classified based on common fundamental features such as cellular arrangement, body symmetry, presence or absence of coelom, and specific features of digestive, circulatory, and reproductive systems.

Levels of Organisation

• Cellular level: cells arranged as loose aggregates, found in Porifera (sponges).
• Tissue level: cells performing the same function form tissues, found in coelenterates.
• Organ level: tissues grouped together to form an organ, which performs a particular function, e.g. Platyhelminthes.
• Organ system level: a few organs coordinatively perform a certain physiological function, e.g. Annelids, Arthropods, Molluscs, Echinoderms, and Chordates.

Circulatory Systems

• Open circulatory system: cells and tissue directly receive the blood pumping out of the heart.
• Closed circulatory system: blood is circulated through arteries, veins, and capillaries.

Body Symmetry

• Asymmetrical: no line of symmetry in the body, e.g. sponges.
• Radial symmetry: any plane passing through the centre divides the body into two symmetrical halves, e.g. coelenterates, ctenophores.
• Bilateral symmetry: a plane divides the body into symmetrical left and right halves, e.g. annelids, arthropods, etc. Echinoderms exhibit radial as well as bilateral symmetry at different stages of their life.

Body Cavity

• Coelom: body cavity between the body wall and gut wall, lined by mesoderm.
• Acoelomates: body cavity is absent, e.g. Platyhelminthes.
• Pseudocoelomates: mesoderm is present as scattered pouches, e.g. Aschelminthes.
• Coelomates: having coelom (body cavity), e.g. from Annelida to Chordata.

Embryonic Development

• Diploblastic: embryo with two germinal layers called external ectoderm and internal endoderm, e.g. Porifera, Cnidaria.
• Triploblastic: embryo with three germinal layers, mesoderm between ectoderm and endoderm, e.g. Platyhelminthes to Chordates.

Body Segmentation

• Metameric segmentation: body shows repeating parts, e.g. earthworm's body.

Chordates

• Animals with notochord are called chordates, e.g. from Chordata.
• Animals without notochord are called non-chordates, e.g. from Porifera to Echinodermata.

Phylum Ctenophora

• Radially symmetrical, diploblastic, and acoelomate
• Eight rows of ciliated comb plates present externally
• Digestion occurs both extracellularly and intracellularly
• Bioluminescence is present
• Hermaphroditic, with external fertilization and indirect development

Phylum Platyhelminthes (Flatworms)

• Mostly endoparasites, dorsoventrally flattened body
• Triploblastic, bilaterally symmetrical, and acoelomate
• Hooks and suckers are present in parasites
• Flame cells help in osmoregulation and excretion
• Hermaphroditic or monoecious, with internal fertilization and indirect development
• Planaria can regenerate

Phylum Aschelminthes

• Free-living or parasitic, aquatic or terrestrial
• Round body in cross-section, bilaterally symmetrical, and triploblastic
• Pseudocoelomate with organ system organization
• Alimentary canal is complete and has a muscular pharynx
• Dioecious, with females longer than males
• Internal fertilization with direct or indirect development

Phylum Annelida

• Bilaterally symmetrical, triploblastic, and coelomate
• Metamerically segmented
• Longitudinal and circular muscles help in locomotion
• Nereis has appendages called parapodia for swimming
• Closed circulatory system
• Nephridia are present for osmoregulation and excretion
• Paired ganglia are connected to a double ventral nerve cord by lateral nerves
• Sexual reproduction, with dioecious, monoecious, and hermaphroditic species