Arthropods and Annelids: Key Biology Notes

Biology I: Arthropods

Arthropods

• Bilateral symmetry
• Segmented body
• Exoskeleton of chitin (impermeable)
• Dehydration prevention; growth bound to ecdysis (molting of the cuticle and periodic shedding)
• Jointed appendages (legs, antennae, palps, etc.)
• Open or lacunar circulation
• Nervous system: ganglionic (nodal), ventral, with varieties of sensory organs (tactile or chemoreceptor antennae, tactile bristles, compound eyes)
• Sexual reproduction or parthenogenesis (development without fertilization; e.g., some crustaceans, bees, fleas, scorpions)
• Most species present metamorphosis
• Gas exchange:
  • Gills (crustaceans)
  • Tracheae (insects, chilopods, diplopods)
  • Book lungs or foliaceous lungs (arachnids)

Main Groups

• Crustaceans (gills): shrimp, crabs, lobsters, pitu, woodlice (garden pill bugs), beach hoppers, crayfish (crawfish)
• Insects (hexapods): flies, cockroaches, and others
• Arachnids: spiders, scorpions, ticks, mites
• Chilopods: centipedes
• Diplopods: millipedes (e.g., snake lice, Embu/Gongola)

Annelids

• Triploblastic body plan with a prostomium
• Segmentation and coelom present
• Bilateral symmetry
• Skeleton absent; simple epithelium
• Complete digestive system; digestion completely extracellular

Physiological Systems

Circulation: Closed circulation; blood is fully channeled within vessels.

Respiration: Cutaneous respiration through the integument, gas exchange by diffusion; in some marine representatives, respiration is branchial.

Nervous system: Nodal type, consisting of several ganglia connected by a ventral nerve cord, with a circumesophageal ring.

Excretion: The excretory system consists of metanephridia; each body segment has one pair of nephridia.

Digestion: Complete digestive system with a tubular mouth and anus; extracellular digestion. It is a fairly specialized system, since there is a great variety of diets.

Integument: Covered by an external cuticle secreted by the epidermis, rich in mucous glands. Subcutaneous nerve endings are present, and the body has a longitudinal muscle system.

Main Groups

• Oligochaeta: live in both terrestrial environments and freshwater. Respiration occurs through the skin. Example: earthworm.
• Polychaetes: most live in marine environments; many dig burrows in sand. Examples: Serpula and Nereis.
• Hirudinea: live mainly in freshwater, though not exclusively. They have suckers at each end of the body. Respiration occurs through the skin. Example: leeches.