Biology: Levels of Organisation in Animals

Study Notes

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

Description

This quiz covers the classification of animals based on their fundamental features, including cellular arrangement, symmetry, and presence of coelom, as well as specific features of digestive, circulatory, and reproductive systems.