Zoology: Invertebrates and Vertebrates
What you must understand
Invertebrates include sponges, coelenterates (radial symmetry, two layers, gastrovascular cavity), flatworms (bilaterally symmetrical, no body cavity), roundworms (pseudocoel, cuticle), annelids (segmented, coelom, closed circulation), molluscs (mantle, shell, radula), arthropods (jointed appendages, chitin exoskeleton, molting; insects with three body regions and tracheal respiration), and echinoderms (five-part radial symmetry, water-vascular system).
Vertebrates share a vertebral column, internal skeleton, and closed circulation. Fish: gills, two-chambered heart, external fertilization. Amphibians: moist skin, lungs plus skin respiration, three-chambered heart, metamorphosis. Reptiles: dry scaly skin, lungs, three-chambered heart (crocodiles four), amniotic eggs. Birds: feathers, four-chambered heart, warm-blooded, lungs with air sacs. Mammals: hair, mammary glands, diaphragm, four-chambered heart, warm-blooded, most with intrauterine development.
Traps
- Arthropods have an open circulatory system; annelids closed.
- Amphibian three-chambered heart mixes blood; birds and mammals have four chambers and full separation.
- Echinoderm adults are radially symmetrical but larvae are bilateral.
- Reptile eggs are amniotic, amphibian eggs are not.
Rule of thumb
Use three filters: body symmetry, body cavity, and respiratory/circulatory grade.
Worked example: An animal with jointed legs, chitin exoskeleton, tracheae and three body regions is an insect - an arthropod with an open circulatory system.