Biology: Invertebrate and Vertebrate Nervous Systems

Invertebrate and Vertebrate Animals

• Invertebrate animals differ from vertebrate animals in many respects
  • Nervous system: invertebrates have a nerve cord that lies ventrally to the digestive tract, while vertebrates have a neutral tube that lies dorsally to the digestive tract
  • Respiratory organs: invertebrates develop from the ectoderm, while vertebrates develop in relation to the pharynx and form gills or lungs
  • Circulatory system: invertebrates have an open system, while vertebrates have a closed system with arteries, veins, and capillaries

Classification of Invertebrates

Phylum Cnidaria

• Class Hydrozoa:
  • Solitary or colonial forms
  • Asexual polyps and sexual medusae (may be suppressed)
  • Freshwater or marine animals
  • Examples: Hydra and Obelia
• Class Scyphozoa:
  • Solitary medusa
  • Polyp stage reduced or absent
  • Medusae do not have velum
  • All marine animals
  • Example: Aurelia
• Class Cubozoa:
  • Solitary medusoid forms
  • Polyp stage reduced
  • Medusa is square in cross-section
  • All marine animals
  • Example: Carybdea
• Class Anthozoa:
  • All are polyps
  • Solitary or colonial
  • Gonads are gastrodermal
  • All are marine animals
  • Example: Sea anemone

Phylum Platyhelminthes

• Characteristics:
  • Bilaterally symmetrical
  • Dorsiventrally flattened (flatworms)
  • Triploblastic animals (made up of three body layers)
  • Acoelomate (lack body cavity)
  • Complete reproductive organs
  • Digestive system is absent in some, with only a mouth and no anus
  • Nervous system is ladder-like, with simple sense organs
  • No respiratory, circulatory, or skeletal system
  • Proto-nephridial type of excretory system

Classification of Platyhelminthes

• Class Turbellaria:
  • Mostly free-living and aquatic, with soft bodies and leaf-like form
  • Body covered with cilia, some are terrestrial and confined to humid areas
  • Examples: Planaria
• Class Trematoda:
  • Parasitic, lacking cilia, cuticle covering leaf-like body with one or more suckers
  • Examples: Faciola hepatica (liver fluke), Schistosoma (blood fluke)
• Class Cestoda:
  • Endoparasites (internal parasites), having no gut (digestive) system
  • Examples: Taenia (tapeworms), Ancyclostoma duodenale (hookworm)

Phylum Annelida

• Characteristics:
  • Mostly aquatic, some are terrestrial
  • Body is vermiform, bilaterally symmetrical, and metamerically segmented
  • Straight tube alimentary canal, with extra-cellular digestion
  • Segmentally arranged locomotory organs, repeated groups of chitinous setae or chaetae
  • Respiration is generally through body surface or through a special projection of parapods
  • Well-developed closed type blood vascular system
  • Nephridia are the excretory organs
  • Nervous system consists of paired cerebral ganglia or brain, a double ventral nerve cord bearing segmental ganglia

Classification of Annelida

• Polychaeta:
  • Mostly marine forms, distinct head with eyes and tentacles
  • Segmental with lateral projection of the body wall (parapodia)
  • Sexes are separate, reproduce asexually by budding
  • Example: Rag worm
• Oligochaeta:
  • Live in soil or freshwater, body is conspicuously segmented but no distinct head
  • Parapodia absent, hermaphrodites, reproductive system is more complicated
  • Clitellum is present, no larva, and development is direct
  • Example: Earthworm
• Hirudinea:
  • Members have fixed body segments numbering 34
  • Some group may have only 31 or 17 segments
  • Examples: Leeches

Phylum Chordata

• Four diagnostic features:
  • Notochord: a structure that runs through the digestive system and its nerve chord
  • Dorsal hollow nerve chord: a tube nerve fiber that develops into the central nervous system
  • Pharyngeal slits: in lancelates, they function as filters for feeding
  • Post-anal tail: helps propel animals in water

Classification of Chordata

• Subphylum Urochordata/Tunicata:
  • Invertebrate chordates, exclusively marine
  • Notochord is present only in larval tail
  • Examples: Ascidia, Salpa, and Doliolum
• Subphylum Cephalochordata:
  • Notochord extends from head to tail region and is persistent throughout life
  • Examples: Branchiostoma (Amphioxus/Lancelets)
• Subphylum Vertebrata:
  • Possess notochord during embryonic period, replaced by a cartilaginous or bony vertebral column in adults
  • Two image-forming eyes, well-developed closed circulatory system, kidneys for excretion and osmoregulation
  • Paired appendages which may be fins or limbs, centralized nervous system, digestive system is complete
  • Examples: Frogs, toads, and salamanders