Read Invertebrates by Their Defining Systems
A phylum is recognised by combinations of neural layout, circulation, movement and specialised structures.
Living-system questions are solved by matching a structure or organism to its function, process and stated condition.
Compare major invertebrate systems
Hydra has a nerve net; annelids have a ventral nerve cord with segmental ganglia; echinoderms have a circumoral nerve ring and radial nerves. Sponges have a canal system; annelids show metameres; arthropods have jointed appendages; echinoderms have a water-vascular system and calcareous ossicles.
Connect structure with function
Spider leg extension uses haemolymph pressure. Echinoderm dermal ossicles and spines explain the name “spiny-skinned.” Aschelminthes are cylindrical, unsegmented and lack a specialised circulatory system; any row describing them as flat or having a circulation system is mismatched. Porifera through Hemichordata are non-chordates.
OlympIQ exam lens
- ✓Hydra nerve net; annelid ventral cord; sea-star radial nerves.
- ✓Spider legs use haemolymph hydraulics.
- ✓Arthropoda: jointed appendages; Echinodermata: water-vascular system.
- ✓Echinoderm ossicles support dermal spines.
Haemolymph pressure is particularly important in spider leg extension; do not confuse it with vertebrate bony support.
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