The skeletal system in invertebrates¹ that have a skeleton is usually external² and is called an exoskeleton, eg³ a hard outer shell, a hard cuticle, etc. Even the simplest animals – sponges – have small, sharp skeletal elements called spicules; echinoderms have an endoskeleton lying under the skin. Chordates have an endoskeleton. In primitive chordates,... Continue Reading →

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