Kingdom Animalia

Animals are

• eukaryotic
• multicellular organisms
• that do not possess a cell wall
• heterotrophs, digest internally
• most are capable of locomotion

Various phyla:

• Cnidaria
• Platyhelminthes
• Annelida
• Anthropoda
• Chordata

• include marine jellyfish and freshwater Hydra

• radially symetrical
• restricted to aqueous medium for support + as have no means of restricting water loss
• hydrostatic skeleton formed by the fluid filled enteron (gut cavity)

Flatworms eg. planarians and river flukes

• Structure:
  • bilaterally symmetrical
  • Flattened dorso-ventrally

An advantage of bilateral symmetry is animals have a 'front' where sensory perceptors can be positioned, so they can 'test' an environment before entering. Streamlined bilaterally symmetrical shape makes movement much easier

• much more tissue than cnidarians
• tube-like gut rather than enteron
  • meaning solid tissue fills the space btwn outer surface and gut
    • this tissue allows specialisation
    • also means many cells per unit volume involved in metabolic activity, meaning high demands for O₂

• Dorso-ventral flattening increases SA:Vol ratio
  • advantages:
    • increases uptake of O₂ due to high SA:Vol ratio
    • decreases diffusion distance from body surface to body cells
• Platyhelminthes have a single opening to the gut
  • means the remains of food are passed out of same body opening through which it enters
• Surrounding aqueous medium provides suppt for most platyhelminthes, but cells btwn outer surface and gut provide support as 'packing tissue'

Radial

• radial symmetry means there is symmetry around a central axis
• no left or right 'side'
• body can be divided in half by any plane through the cross section ie. organism is round in cross-section
• Sessile animals show radial symmetry, allowing them to respond to environment in all directions

Bilateral 

• Bilateral symmetry means that the body can be divided into two equal halves on each side of a central axis- there are left and right 'sides'
• The two halves are mirror images of each other
• Associated with animals that move through their environment to search for food

• round worms eg. earthworms and lugworms
• more rounded in cross-section compared to platyhelminthes
• Bilaterally similar
• Have a coelom - spaces within the tissue that lies btwn body surface and the gut
• Chatae- external bristles of chitin that help w/ ease of mvmt
• Cylindrical shape provides large SA:Vol ratio
  • Earthworms have a fairly low level of activity ∴ low rate of O₂ consumption, so don't need special organs for gas exchange
• Advantages of possessing a coelom
  • ratio of SA:metabolically active tissue is increased
    • why they're round- not same need to maximise SA:vol ratio
  • can function as effective hydroskeleton, as fluid filled
  • muscles involved in locomotion separated from gut muscles
    • allows mvmt of organism and peristaltic gut mvmts to occur independently
  • provides room for development of organs

• metamerically segmented- body is divided into large no. of structurally similar segments, each w/ own body cavity
• Annelids have a blood system and simple nervous system
• One-way gut w/ separate mouth and anus
  • allows regional specialisation
  • prevents food waste being mixed up w/ incoming food
  • w/in the gut there is 
    • a muscular pharynx
    • an oesophagus
    • a crop (storage area)
    • a muscular gizzard (for mechanical absorption)
    • an intestine for absorption

eg. insects and spiders

• bilaterally symmetrical
• typically have fixed no. of metameric segments in each region of body eg. head, thorax and abdomen for insects - 3 in rhorax, 10-11 in abdomen
• jointed limbs
• both a mouth and an anus
• gut shows regional specialisation

arachnids

• bilaterally similar and have fixed no. of metameric segments,
• however have 4 pairs of legs rather than 3
• body has 2 main sections rather than 3 (head + thorax combined - cephalothorax)

• Anthropods, particularly insects are most successful animal group on earth
• as basic insect body plan can be easily adapted evolutionarily to fill wide range of niches
  • eg. mouthparts evolved,
  • have wings,
  • several distinct body forms (like larval) w/ separate food sources

• Vertebrates are chordates
• main groups are the fish, amphibians, reptiles and mammals
• bilaterally symmetrical and segmented
• body cavity much greater proportionally in vertebrates
  • + more continuous than in annelids
  • contains much more extensively developed and complex organs organised into complex systems eg. digestive, circulatory
• there is a vertebral (spinal) column
  • w/ segmented muscle blocks
  • w/ an endoskeleton consisting of an internal jointed system of calcified bones
•  One-way gut w/ a mouth and anus
  • high degree of regional specialisation along the length of the gut

Note: segmentation is a linear series of repeating units

most obvious in annelids and arthropods

more subtle in chordates, includes vertebrae and rib bones

NOTE: there are a very small group of non-vertebrate chordates eg. lancelets

Evolutionarily they are a bridge btwn invertebrates and vertebrates, with a stiff dorsal rod rather than a true backbone

The sequence from Cnidarians→Platyhelminthes→Annelida→Arthropoda→Chordata shows no. of evolutionary trends including:

• Gradiation from radial to bilateral symmetry
• Gradiation from solid tissue btwn body surface and the gut lining to the presence of cavities (that reduce the amnt of metabolically active tissue per unit volume)
• Development of a one-way gut as opposed to a gut cavity (cnidarians) or gut with only one opening (platyhelminthes)