Q.Give an account of the salient features of phylum Arthropoda, and explain why it is considered the most successful and diverse phylum in the Animal Kingdom.

Step 1. Body plan. Arthropoda is bilaterally symmetrical, triploblastic and coelomate, with an externally segmented body whose segments are grouped into functional regions (head, thorax, abdomen, or a fused cephalothorax and abdomen).

Step 2. Defining feature. The phylum's name-giving feature is its jointed appendages — legs, antennae and mouthparts that articulate at each joint, permitting very precise, localised movement, an advantage over the less differentiated appendages of, say, Annelida.

Step 3. Exoskeleton. The body is protected by a hard, chitinous exoskeleton that also anchors muscles, but must be periodically shed and replaced (moulting/ecdysis) to allow growth — a metabolic cost, but one that buys strong protection against predation and desiccation.

Step 4. Circulation and respiration. Circulation is of the open type (haemolymph in a haemocoel), a comparatively simple but adequate system; respiration is highly adaptable to habitat — gills in aquatic forms, book lungs in many arachnids, and an extremely efficient tracheal system in insects that delivers oxygen almost directly to the tissues.

Step 5. Diversity across classes. These adaptable features are expressed across four major classes — Crustacea (mostly aquatic, e.g. Palaemon), Arachnida (e.g. scorpions, spiders), Insecta (by far the largest class, occupying nearly every terrestrial and fresh-water niche, e.g. Apis, Bombyx, Anopheles) and Myriapoda (centipedes, millipedes) — illustrating an exceptional range of body forms on one basic plan.

Step 6. Why most successful. Taken together — a versatile jointed body plan, a protective yet moultable exoskeleton, efficient and habitat-specific respiration, and small body size relative to resource needs — these features let arthropods, and insects above all, exploit an enormous range of ecological niches; this is reflected in species numbers alone, since insects account for over two-thirds of all described animal species and Arthropoda as a whole outnumbers every other phylum combined.

Cover body plan, the defining jointed-appendage feature, the exoskeleton/moulting trade-off, circulation and respiration, and class-level diversity, then tie these features explicitly to the phylum's species-level dominance.

• Listing the salient features without ever explicitly connecting them to why the phylum is so diverse — the question specifically asks for that causal link, not just a features list.
• Omitting the moulting requirement when describing the exoskeleton, which is an essential limitation/trade-off of having a rigid, non-growing outer covering.