Kingdom Animalia includes all multicellular, heterotrophic animals. This chapter first defines the important terms used in describing and classifying animals, then surveys the major animal phyla from Porifera to Chordata, with their general characters, classification and examples.
Important terms used in the characters and classification of organisms
1) Cellularity
Organisms are made up of cells, the basic units of life. They may be of two types:
- Unicellular: made up of only one cell, e.g. protozoans.
- Multicellular: made up of many cells, also called metazoans, e.g. all organisms except protozoans.
2) Body layer
This refers to the cellular layers that form the covering of the body.
- Diploblastic: animals derived from two germ layers, outer ectoderm and inner endoderm, e.g. sponges.
- Triploblastic: animals derived from three germ layers, outer ectoderm, middle mesoderm and inner endoderm.
3) Body organisation and surfaces
- Anterior end: the part usually containing the mouth.
- Posterior end: the part usually containing the anus.
- Dorsal surface: the upper surface, facing the sky in the natural posture.
- Ventral surface: the lower surface, facing the ground in the natural posture.
- Laterally compressed: a body flattened from both lateral sides.
4) Symmetry
- Bilateral symmetry: the body can be divided into two exactly equal and similar halves.
- Radial or biradial: the body, cut from any side passing through the central axis, divides into two equal halves.
- Asymmetry: the body cannot be cut into two equal halves.
5) Habitat
- Aquatic: organisms that live only in water; either fresh-water or marine.
- Amphibious: animals that can live both on land and in water.
- Terrestrial: animals that live only on land.
6) Body temperature
- Cold-blooded (poikilothermic): body temperature varies with the environment.
- Warm-blooded (homeothermic): body temperature does not vary with the environment.
7) Coelom (body cavity)
The coelom is the cavity found between the body wall and the wall of the alimentary canal.
8) Locomotion
The process of movement of an organism from one place to another. The locomotory organs in various animals are:
- Pseudopodia: cytoplasmic processes found in Amoeba.
- Flagella: long, whip-like cytoplasmic processes found in Euglena.
- Cilia: small cytoplasmic processes found in Paramecium.
- Setae: locomotory organs of earthworms.
- Parapodia: locomotory organs of annelids (polychaetes).
- Appendages: locomotory organs of arthropods.
- Muscular foot: locomotory organ of molluscs.
- Tube feet: locomotory organs of starfish.
- Fins: locomotory organs of fish.
- Limbs: locomotory organs of vertebrates.
9) Nutrition
The process of obtaining food.
- Holozoic: food is taken in solid form (animal-like, i.e. heterotrophs).
- Holophytic: plant-like nutrition.
- Saprozoic: feeding on dead organic matter, e.g. earthworm.
- Saprophytic: plants feeding on dead organic matter, e.g. fungi.
- Parasitic: organisms that live on other organisms and remain alive at the expense of the host, e.g. tapeworm.
- Carnivorous: feeding on animal flesh.
- Herbivorous: feeding on plants.
- Omnivorous: feeding on plants as well as animals.
10) Digestion
The process of breaking down complex foods chemically and mechanically.
- Intracellular digestion: takes place inside the cell.
- Extracellular digestion: takes place outside the cell.
11) Respiration
The process of taking in oxygen to oxidise simpler foods in order to release energy; an energy-releasing process.
- Aerobic respiration: takes place in the presence of oxygen.
- Anaerobic respiration: takes place in the absence of oxygen.
12) Circulation
The process of flow of fluid (blood) within the body.
- Open circulation: blood remains in large open sinuses.
- Closed circulation: blood flows through closed blood vessels.
Types of heart
- Neurogenic heart: found in all invertebrates except Echinodermata.
- Myogenic heart: found in Echinodermata and all Chordata.
13) Excretion
The process of removal of waste substances from the body. The excretory organs in various organisms are:
- Contractile vacuole: in protozoans.
- Flame cell: in helminths.
- Nephridia: in annelids.
- Malpighian tubule: in arthropods.
- Organ of Bojanus: in molluscs.
- Kidney: in vertebrates.
14) Reproduction
The process of producing young ones.
- Asexual reproduction: takes place by one parent only.
- Sexual reproduction: male and female sexes are involved, and male and female gametes fuse to form a zygote.
15) Fertilization
The process of fusion of male and female gametes, resulting in the formation of a zygote.