Animal Kingdom - Arthropoda

Arthropoda is identified in NCERT Class 11, Chapter 4 (Animal Kingdom) as the largest phylum, embracing insects, crustaceans, arachnids and myriapods. Its name literally means jointed legs, and the defining features are jointed appendages together with a hard chitinous exoskeleton covering the segmented body. These adaptations permit efficient locomotion, protection against predators and desiccation, and the exploitation of diverse habitats, which collectively explain the group's evolutionary success. Therefore jointed appendages with a chitinous exoskeleton is the correct combination. Soft moist skin with closed circulation is incorrect, because arthropods have a tough cuticle and an open circulatory system with a haemocoel. Radial symmetry and a water vascular system are features of Echinodermata such as starfish, not of arthropods, which are bilaterally symmetrical. A naked body lacking segmentation is the opposite of the arthropod condition, since their bodies are conspicuously segmented and armoured. Arthropods also bear sensory compound eyes, breathe through tracheae or gills, and must moult, or shed the rigid exoskeleton, in order to grow. A consistency check confirms the logic: the very name and the largest-phylum status both point to jointed limbs and a protective chitin exoskeleton as the chief reasons for arthropod diversity and dominance.