Provide the appropriate technical term for the following descriptions:
$(a)$ Blood-filled cavity in arthropods.
$(b)$ Free-floating form of cnidaria.
$(c)$ Stinging organ of jellyfish.
$(d)$ Lateral appendages in aquatic annelids.

(N/A) Haemocoel: The body cavity of arthropods is filled with blood,known as haemocoel.
$(b)$ Medusa: It is the free-swimming,umbrella-shaped form of cnidarians,such as jellyfish.
$(c)$ Nematocysts: These are specialized stinging cells (cnidoblasts) containing a coiled thread used for defense and prey capture.
$(d)$ Parapodia: These are lateral,fleshy,paddle-like appendages found in aquatic annelids like Nereis,which help in swimming.