Question

Name the largest group of animals. Write the salient features of this group. GIVE TWO Examples..............

Answer 1

Arthropoda (Arthon- Jointed; podos- legs)

Arthropoda, animals having jointed appendages or legs.It is a phylum of triploblastic, haemocoelomic, segmented invertebrates having head, thorax and abdomen, a chitinous exoskeleton and jointed legs and appendages.Most successful phylum on the Earth that has ever existed.Arthropods were first studied by Aristotle.Von Siebold coined the term Arthropoda.

 

General characters

Cosmopolitan in distribution found in aquatic, terrestrial and aerial forms. 
Some are ectoparasitic and vectors of disease.Body have jointed appendages or legs (which are modified to different structures to perform different functions like jaws, gills, walking legs, paddle). There may be 3 pairs, 4 pairs, 5 pairs, many pairs.Body is triploblastic.Bilaterally symmetrical.Organ system level of organization.Body is divisible into head, thorax and abdomen.
NOTE: In some (crustacean and arachnida) body is divisible into cephalothorax (head and thorax is fused) and abdomen.This is the first group to develop a true head, which contains sense organs and feeding organs specialized for their particular habitats.Body is covered with chitinous exoskeleton.They are haemocoelomate. Coelom i.e. body cavity is filled with blood or fluid.Head bears a pair of compound eyes and antenna.Locomotion takes place by jointed appendages.Digestive system is complete, straight and well developed.
The mouth bears mouth parts for ingestion of foods. Mouths are modified for chewing, biting, sponging, piercing, siphoning.Respiration takes place by general body surface or gills (in Crustaceans) or trachea ( in insects, diplopoda and chilopoda) or booklungs (Arachnida) and book gills (in king cobra).Circulatory system is of open type i.e. do not have blood vessels and enters directly into the body chambers. The blood is colorless.Excretion takes place through Malphigian tubules (in terrestrial form) or green glands or coxal glands (in aquatic forms).
NOTE: Aquatic forms are ammonotelic, terrestrial forms are uricotelic.Nervous system is of annelidian type, which consists of brain and ventral nerve cord.Unisexual i.e. sexes are separate.Fertilization is internal or external.They are either oviparous or ovoviviparous.Development may be direct or indirect.Sensory organ include antennae, sensory hairs for touch and chemoreceptor, simple and compound eyes, auditory organs (in insects) and statocysts (in crustacean).

Examples

Zoological nameCommon name
Limulus. Horse shoe crab or King crab
Daphnia. Water flea
Cancer. Crab
palaemon. Prawn
Astacus. Cray fish