Complex sentence: Ostracods are small crustaceans, typically around one mm in size, but varying between 0.2 to 30 mm, laterally compressed and protected by a bivalve-like, chitinous or calcareous valve or "shell".
Split into simpler sentences: Ostracods are small crustaceans, typically around in size, but varying between. Their bodies are flattened from side to side and protected by a bivalve-like, chitinous or calcareous valve or "shell".