Biology · Animal Kingdom · NEET
NCERT states that over two-thirds (more than 66%) of all named animal species on Earth are arthropods. That is why Arthropoda is the largest phylum. It includes insects, spiders, scorpions, crabs, prawns, centipedes and millipedes. For NEET, just remember: Arthropoda = largest phylum; Mollusca = second largest phylum.
Remember these in order: (1) organ-system level of organisation, (2) bilateral symmetry, (3) triploblastic, (4) segmented body, (5) true coelomate. The body has head, thorax and abdomen. Key exam points: chitinous exoskeleton, jointed appendages, open circulatory system, respiration by gills/book lungs/tracheal system, and excretion by malpighian tubules.
Arthropoda is a TRUE coelomate. It is bilaterally symmetrical, triploblastic, segmented and coelomate. Do not confuse it with Aschelminthes (roundworms), which are pseudocoelomates. This is a common NEET trap in "true coelomate + segmented" match questions, where the correct groups are Annelida, Arthropoda and Chordata.
Both are segmented and bilaterally symmetrical, but the appendages are different. Annelids (like Nereis) have PARAPODIA, which are lateral fleshy swimming flaps. Arthropods have JOINTED appendages (legs), NOT parapodia. Also, arthropods have a hard chitinous exoskeleton, book lungs or tracheae, and malpighian tubules, which annelids lack. NEET has directly asked that parapodia are NOT an arthropod feature.
Arthropods have an OPEN circulatory system. This means blood (haemolymph) is not always inside closed blood vessels; it flows into open body spaces called sinuses. This matters for NEET because open circulation links Arthropoda with Mollusca, while Annelida and Chordata have closed circulation.
The exoskeleton of arthropods is made of CHITIN, a tough nitrogen-containing polysaccharide. NEET 2022 asked this directly: the answer is chitin, not cutin (plant), not cellulose (plant), and not glucosamine (that is only the building-block monomer, not the polymer).
Respiration happens through gills (aquatic forms like prawns), book lungs (arachnids like scorpions and spiders), or a tracheal system (insects). Excretion is by MALPIGHIAN TUBULES. In NEET match-the-column questions, Bombyx (silkworm) is linked to malpighian tubules and scorpion is linked to book lungs.
Which of the following features is not present in the Phylum Arthropoda?
Exoskeleton of arthropods is composed of:
Consider the following features: (a) Organ system level of organisation (b) Bilateral symmetry (c) True coelomates with segmentation of body. Select the correct option of animal groups which possess all the above characteristics.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Arthropoda is the largest phylum in the animal kingdom because over two-thirds of all named species on Earth are arthropods. Mollusca is the second largest.
Arthropoda comes from Greek: arthros means joint and poda means feet or appendages. So arthropods are "jointed-legged" animals, which is their defining feature.
Economically important insects: Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect). Vectors: Anopheles, Culex, Aedes (mosquitoes). Gregarious pest: Locusta (locust). Living fossil: Limulus (king crab).
Arthropods are mostly dioecious (separate sexes), fertilisation is usually internal, and they are mostly oviparous (egg-laying). Development may be direct or indirect.
Limulus, the king crab (horseshoe crab), is the living fossil mentioned in NCERT for phylum Arthropoda.
