Lesson 6 of 12 · 7 min
Arthropods and molluscs
NCERT §4.2.7, §4.2.8
Back at the fish market, Ammu's basket fills up fast: prawns and crabs from one stall, squid, cuttlefish and oysters from the next. Mosquitoes whine round the ice boxes. Over half her notebook's species are now from just two phyla.
The lesson in notes
In short
Arthropoda, the phylum that holds the insects, is the largest in the animal kingdom: more than two-thirds of the species named on earth belong to it.
Arthropods are bilateral, triploblastic, segmented, coelomate and at the organ-system level. A chitinous exoskeleton covers a body of head, thorax and abdomen, with jointed appendages (arthros, joint; poda, appendages).
They breathe with gills, book gills, book lungs or a tracheal system. Circulation is open. Sense organs include antennae, compound and simple eyes, and statocysts for balance. Malpighian tubules carry out excretion.
Most are dioecious and oviparous; fertilisation is usually internal and development direct or indirect.
Arthropod examples by role. Useful insects: the honey bee Apis, the silkworm Bombyx and the lac insect Laccifer. Disease vectors: the mosquitoes Anopheles, Culex and Aedes. A gregarious pest: the locust Locusta. A living fossil: the king crab Limulus.
Mollusca is the second largest animal phylum. Molluscs live on land or in water, marine or fresh, and are bilateral, triploblastic, coelomate and at the organ-system level.
The unsegmented body has a distinct head, a muscular foot and a visceral hump, and is covered by a calcareous shell. A soft, spongy skin fold, the mantle, covers the hump; the mantle cavity between them holds feather-like gills that serve both respiration and excretion.
Sensory tentacles sit on the head, and a file-like rasping organ, the radula, in the mouth is used for feeding. Most molluscs have separate sexes, lay eggs and develop through a larva. Examples: the apple snail Pila, the pearl oyster Pinctada, the cuttlefish Sepia, the squid Loligo, Octopus (devil fish), the sea-hare Aplysia, the tusk shell Dentalium and the chiton Chaetopleura.