Structural Organisation in Animals
Structural Organisation in Animals asks how the body plan of representative animals is built from tissues and organs. The exam favours earthworm, cockroach and frog because each shows a different solution to support, movement, gas exchange and circulation.
Core rules
- Earthworm: metameric coelomate; clitellum in segments 14-16; setae for locomotion; nephridia for excretion; closed circulation with dorsal vessel and lateral hearts; typhlosole increases intestinal surface; calciferous glands regulate calcium.
- Cockroach: chitinous exoskeleton; head, thorax and abdomen; three pairs of walking legs; two pairs of wings; tracheal system; Malpighian tubules; open circulation with dorsal heart; compound eyes.
- Frog: moist permeable skin; three-chambered heart; ureotelic; external fertilisation; metamorphosis.
Worked example
A specimen has a clitellum over segments 14-16, four pairs of setae per segment, nephridia and a closed circulatory system. Identify it and justify excluding leech and Ascaris.
Reasoning: clitellum and setae are annelid features; closed circulation and nephridia place it among oligochaetes, the earthworm. Leech is an annelid but has suckers and no setae. Ascaris is a pseudocoelomate roundworm with no setae or clitellum. Answer: earthworm.
Traps that cost marks
- Cockroach blood is not the oxygen carrier; tracheae deliver oxygen directly.
- Earthworm is not parasitic; its clitellum secretes a cocoon for eggs.
- Frog has a three-chambered heart, not four.
- Do not confuse Malpighian tubules for excretion with tracheae for respiration.