a. Blood-filled cavity in arthropods: Haemocoel

Explanation: Haemocoel is the blood-filled cavity in arthropods containing haemolymph

b. Free-floating form of cnidaria: Medusa

Explanation:

Medusa is a form in cnidarians in which the body is shaped like an umbrella which can float freely in sed water.

c. Stinging organ of jelly fishes: Nematocyst

Explanation:

Nematocytes are capsules of specialized cells in cnidarians which act as a paralyzing sting.

d. Lateral appendages in aquatic annelids: Parapodia

Explanation:

Parapodia are the paired lateral outgrowth in annelids which are not joint and bearing chaetae.