(a) Blood – filled cavity in arthropods Haemocoel
Haemocoel is the blood filled cavity in Arthropods and molluscs, it helps to transport nutrients, gases and excretion of waste.
Arthropods possess open circulatory system and lacks blood vessels and thus cells and tissues directly bathed in the haemolymph.
(b) Free – floating form of cnidaria Medusa
Phylum Cnidaria exhibits two basic body forms called as Polyp and Medusa respectively.
Polyp form is sessile and cylindrical example as observed in Hydra.
Medualls form is umbrella-shaped and free swimming for example as observed in Jelly fishes.
(c) Stinging organ/cell of jellyfishes Cnidocytes
An important characteristic feature of phylum Cnidaria is the presence of stinging cells or cnidocytes.
These are specialised neural cells found in Jelly fishes, corals, sea anemones to capture food and for defence.
d) Lateral appendages in aquatic annelids Parapodia.
The body surface of members of phylulm Annelida is metamerically segmented, possess longitudinal and circular muscles for locomotion.
Aquatic forms like Nereis, possess parapodia for locomotion.
Final Answer
(a) Blood – filled cavity in arthropods Haemocoel
(b) Free – floating form of cnidaria Medusa
(c) Stinging organ/cell of jellyfishes Cnidocytes
d) Lateral appendages in aquatic annelids Parapodia.