In arthropods like insects and crustaceans, an internal ridge or projection of the exoskeleton that serves as an attachment point for muscles.
From Greek 'apo-' (away, off) + 'deme' (a bond or bundle), referring to internal structures that branch off from the main skeleton to support muscle attachment.
Insects and crustaceans have a brilliant engineering solution—instead of internal skeletons like ours, they have internal ridges on their outer shells that muscles can grab onto, making them incredibly efficient movers.
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