Animal Kingdom Classification

Fundamental terminology and characteristics for animal kingdom classification including levels of organisation, symmetry, body cavities, and phylum-specific features.

Cellular level of organisation

A pattern of organisation where cells are arranged as loose cell aggregates, as seen in sponges.

Tissue level of organisation

A level of organisation in which cells performing the same function are arranged into tissues, exhibited by coelenterates.

Organ level of organisation

A level where tissues are grouped together to form organs specialized for a particular function, exhibited by Platyhelminthes.

Organ system level of organisation

An organisation where organs associate to form functional systems concerned with specific physiological functions, found in Annelids, Arthropods, Molluscs, Echinoderms, and Chordates.

Incomplete digestive system

A digestive system with only a single opening to the outside of the body that serves as both mouth and anus.

Open circulatory system

A system in which blood is pumped out of the heart and the cells and tissues are directly bathed in it.

Closed circulatory system

A system in which blood is circulated through a series of vessels of varying diameters, such as arteries, veins, and capillaries.

Radial symmetry

A body plan where any plane passing through the central axis of the body divides the organism into two identical halves, seen in coelenterates, ctenophores, and echinoderms.

Bilateral symmetry

A body plan where the body can be divided into identical left and right halves in only one plane, exhibited by annelids and arthropods.

Diploblastic

Animals in which the cells are arranged in two embryonic layers: an external ectoderm and an internal endoderm.

Mesoglea

An undifferentiated layer present between the ectoderm and the endoderm in diploblastic animals.

Triploblastic

Animals in which the developing embryo has a third germinal layer, the mesoderm, between the ectoderm and endoderm.

Coelom

A body cavity lined by mesoderm situated between the body wall and the gut wall.

Pseudocoelom

A body cavity where the mesoderm is present as scattered pouches between the ectoderm and endoderm rather than a continuous lining.

Acoelomates

Animals in which the body cavity is absent, such as Platyhelminthes.

Metamerism

A phenomenon of segmentation where the body is externally and internally divided into segments with a serial repetition of at least some organs.

Notochord

A mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals.

Choanocytes

Also called collar cells, these line the spongocoel and canals in sponges to assist in water transport and food gathering.

Cnidoblasts

Cells containing stinging capsules (nematocysts) used for anchorage, defense, and capture of prey in Cnidarians.

Metagenesis

The alternation of generation seen in some cnidarians, where polyps produce medusae asexually and medusae form polyps sexually.

Bioluminescence

The property of a living organism to emit light, which is well-marked in ctenophores.

Flame cells

Specialised cells in Platyhelminthes that help in osmoregulation and excretion.

Radula

A file-like rasping organ for feeding found in the mouth of most molluscs.

Water vascular system

A distinctive system in echinoderms that helps in locomotion, capture and transport of food, and respiration.

Stomochord

A rudimentary structure in the collar region of hemichordates that is similar to a notochord.

Poikilothermous

Cold-blooded animals that lack the capacity to regulate their body temperature, such as Chondrichthyes, Osteichthyes, Amphibia, and Reptilia.

Homoiothermous

Warm-blooded animals that are able to maintain a constant body temperature, such as Aves and Mammalia.

Operculum

A gill cover found in Osteichthyes that protects the four pairs of gills.

Air bladder

An internal organ in Osteichthyes (bony fishes) that regulates buoyancy.

Pneumatic bones

Long bones that are hollow with air cavities, a characteristic feature found in Aves (birds).