Q.Provide appropriate technical term in the space provided. a. Blood-filled cavity in arthropods ______________.
b. Free-floating form of cnidaria ______________.
c. Stinging organ of jelly fishes ______________.
d. Lateral appendages in aquatic annelids ______________.

a. Blood-filled cavity in arthropods — haemocoel, the space directly bathed by blood in their open circulatory system.

b. Free-floating form of cnidaria — medusa.

c. Stinging organ of jellyfishes — nematocyst, held within the cnidoblast. …

Each blank names a specific structure described for its respective group.

• a. Blood-filled cavity in arthropods: arthropods have an open circulatory system, in which blood pumped from the heart directly bathes the cells and tissues rather than staying confined to vessels; the space through which this blood flows and bathes the tissues is the haemocoel.
• b. Free-floating form of cnidaria: cnidarians occur in two forms, the sessile polyp and the free-swimming, umbrella-shaped medusa, as seen in Aurelia (jellyfish); the free-floating form is the medusa. …

Method: Word-root / functional-derivation approach

Rather than recalling each technical term directly from memory, derive each term from its function or its word parts, treating the blank as a puzzle to be reasoned out rather than a fact to be recalled whole.

a. Blood-filled cavity in arthropods. Arthropods have an OPEN circulatory system, meaning blood is not confined to vessels but directly bathes the tissues. The Greek root "haema-" relates to blood, and "-coel" relates to a cavity — combining them gives "haemocoel," literally a blood-filled cavity, which is exactly the space through which blood flows and bathes the tissues in an open system.

b. Free-floating form of cnidaria. Cnidarians occur in two forms: a fixed, attached polyp, and a form that floats/swims freely, shaped like an umbrella (as in the familiar jellyfish, Aurelia). The free-floating form specifically has its own name, distinct from "polyp": medusa.

c. Stinging organ of jellyfishes. Jellyfish bear stinging cells (cnidoblasts/cnidocytes) on their tentacles, and each such cell houses a small capsule that actually fires the sting — this capsule, not the cell itself, is the precise stinging ORGANELLE/organ being asked for: the nematocyst. …