• Excretory product is ammonia/urea in aquatic forms and uric
acid in terrestrial species.
nervous system
• Nervous system is ladder-like i.e., annelidan type. It has
paired preoral ganglia (cerebral and suboesophageal),
connectives (longitudinal nerve bands), commissures
(lateral nerve bands) and a double ventral nerve cord
having ganglia and nerves. A pair of ganglia are found at
each segment.
• Antennae are absent in arachnids e.g., spiders etc. Antennae
are two pairs in crustaceans e.g., freshwater prawn etc., and
single pair in others e.g., centipede (Scolopendra), millipede
(Julus), cockroach etc.
• The sense organs comprise chemo and tactile receptors. In
many arthropods, compound eyes forming mosaic vision are
well developed. A balancing organ called statocyst is also
present in some.
reproductive system
• Sexes are usually separate (dioecious) and sexual
dimorphism is observed in many forms. Reproductive
organs and ducts are paired.
• Fertilisation is usually internal. The animals are
oviparous and occasionally ovoviviparous e.g., scorpion.
Eggs are macrolecithal, cleidoic and mostly centrolecithal.
• Development may be direct or indirect with many larval
stages which undergo metamorphosis to produce the adult
form.
• Some forms show parthenogenesis i.e., development of
an egg into a complete individual without fertilisation e.g.,
male honey bees (drones).
• The level of maternal care for hatchlings varies from non-
existent to the prolonged care provided by scorpions.
larvae of crustaceans
• Nauplius Characteristic larvae of the
class crustacea
• Metanauplius Apus
• Protozoaea Euphausia
• Zoaea Crabs
• Megalopa Brachyuran decapods
• Phyllosoma Palinurus
• Alima Squillalarvae of insects
• Maggots or gentles Musca domestica
• Wriggler Mosquitoes
• Grubs Honey bee, beetles
• Caterpillar Silkwormlarvae of arthropods
unique features
• jointed appendages modified for a variety of functions.
• Tough jointed exoskeleton of chitinous plates.
• Trachea, book lungs, book gills, etc. for respiration.
• Antennary glands or green glands, Malpighian tubules and
coxal glands for excretion and osmoregulation.
• Presence of compound eyes in most forms.
• Body cavity is haemocoel and blood is called
haemolymph.
• Secretion of pheromones (a volatile chemical substance)
in some forms.
• Power of flight in most insects.advanCement over annelida
• Well developed head, distinct from other regions of body.
• Well developed chitinous exoskeleton.
• Jointed appendages adapted for different functions.
• Striped muscles arranged in bundles for moving particular
parts.Annelids have somatic musculature in the form of
continuous muscular layers. In arthropods, on the
other hand, due to much jointedness of body and limbs,
the musculature consists of separate striated muscles,
attached to the inner surface of the exoskeleton. The jointed
exoskeleton on body and appendages, with its associated
discontinuous muscles provides for a great variety of
specialised movements. Each joint has flexor muscles
which bend it, and extensor muscles which straighten it
out.advantage of discontinuous muscle bundles• Well developed sense organs e.g., compound eye, statocyst,
taste receptor, sensory hair etc.
• Development of endocrine glands for secretion of
hormones.
• Secretion of pheromones for communication.
• Special respiratory organs such as trachea, book gills
and book lungs which were absent in annelids.
• Development of eyes (simple and compound eyes) which
were not so well marked in annelids.ClassifiCation of arthroPoda
• Arthropoda is a much heterogenous group. The versatility
of the arthropod modular body plan has made it difficult
for zoologists to classify them. Because of this, no definitive