Animal Biology: Structure, Nutrition, Respiration, Circulation and Reproduction
This domain compares animal groups by how they solve the same problems - getting food, oxygen and blood around the body - and by their reproductive strategies.
Feeding and digestion
Cnidarians have a gastrovascular cavity with one opening. Annelids, insects and vertebrates have a complete gut with mouth and anus, allowing one-way flow and regional specialisation. Digestion may be intracellular or extracellular; saprotroph-style absorption does not occur in animals.
Respiration and circulation
- Gills: fish, counter-current exchange of gases with water.
- Tracheae: insects - air tubes opening at spiracles, delivering gas directly to tissues; hence insects need no blood pigment for oxygen.
- Lungs: amphibians, reptiles, birds, mammals; alveoli give a large surface.
- Skin: earthworms supplement. Circulation is open in insects (haemolymph bathes organs, no capillaries) and closed in annelids and vertebrates. Fish have single circulation (heart → gills → body); amphibians and reptiles have double circulation with incomplete separation; birds and mammals have complete double circulation with four chambers.
Reproduction
External fertilisation (most fish, amphibians) needs water and produces many gametes; internal fertilisation (reptiles, birds, mammals) is independent of water. Development may be direct or indirect with metamorphosis (tadpole → frog; caterpillar → butterfly). Ectotherms rely on the environment for heat; endotherms generate it.
Worked example
A large insect is described as having spiracles along the abdomen and haemolymph that contains no oxygen-carrying pigment. Explain why its size is limited. Reasoning: oxygen reaches tissues through tracheae by diffusion; diffusion is efficient only over short distances; the open circulatory system cannot rapidly transport oxygen; so body size and activity are constrained. Each link must be stated - the examiner wants the diffusion argument, not just "insects are small".
Traps
Claiming insects have closed circulation; saying fish have double circulation; confusing metamorphosis with direct development; forgetting that internal fertilisation is not the same as internal development; describing ectotherms as "cold-blooded" without explaining heat comes from the environment.