• Animals are?multicellular,?eukaryotic?organisms
• They are?heterotrophs?and most have a?nervous system?that enables responses to the surrounding environment
• There are over?30 animal phyla, some of which are?vertebrates?and some of which are?invertebrates
  • Vertebrates have a backbone, or spinal column, while invertebrates do not
• Some of the major animal phyla include:
  • Porifera
  • Cnidaria
  • Platyhelminthes
  • Annelida
  • Mollusca
  • Arthropoda
  • Chordata

Porifera

• Simple?aquatic organisms?consisting of a?colony of cells
  • Commonly known as the?sponges
  • Once thought to be plants
• Two?layers of cells surround an internal cavity
  • They?do not have a mouth?for taking in food,?or an anus?for egestion
• Water is drawn in through?pores?in the outer layer of cells, before?suspended particles from the water?are filtered out and taken up by surrounding cells?for food
  • Porifera are?filter feeders
• There is?no symmetry?in the body plan of the porifera
• The?structure?of poriferans is supported by calcium or silica formations known as?spicules

Porifera take in water through pores for filter feeding

Cnidaria

• Aquatic organisms?also known as the coelenterates
• Their body cavity is surrounded by?two layers of cells?and has a?single opening for both ingestion and egestion
• There are?two distinct body forms?present in the cnidaria phylum
  • Polyps, or hydroids are?sessile?e.g. coral and anemones
  • Medusae?(singular?medusa)?float in the water e.g. jellyfish
  • Some cnidaria go through?both the polyp and medusa?body form during their?life cycle
• Polyps are?filter feeders, while medusae?catch prey?using?stinging cells?on their tentacles
• There is?radial?symmetry?in the body plan of the cnidaria
• Most cnidaria have?soft bodies?that rely on the surrounding water for support, but?corals secrete an exoskeleton made of calcium carbonate

Cnidaria have two distinct body forms; polyps and medusae

Platyhelminthes

• Flat-bodied, unsegmented?worms that include the?flatworms, tapeworms,?and?flukes
• Three layers of cells?surround a branched gut with?one opening for both ingestion and egestion
• Platyhelminthes can be?free-living predators?or?scavengers, or?parasites?that feed on their hosts
• The flat body shape of platyhelminthes means that the?diffusion distance?to all cells is?short, and?no circulatory system?is present
• There is?bilateral?symmetry?in the body plan of the?platyhelminthes
• Platyhelminthes have?soft bodies?with?no skeletal support

The body structure of a member of the phylum Platyhelminthes

Annelida

• Round-bodied, segmented?worms that include the?earthworms?and the?leeches
• Each?ring-shaped body segment?contains a repetition of the structures of blood vessels and nerves
• The?gut?has separate openings for the?mouth?and?anus
• Annelida can be?free-living predators?or?scavengers, or?parasites?that feed on their hosts
• Annelida have a?closed circulatory system
• There is?bilateral?symmetry?in the body plan of the?annelida
• Annelida have?soft bodies?which are supported by a?hydroskeleton
  • The hydroskeleton is an internal fluid-filled cavity called the coelom, against which the muscles of the body wall exert pressure

The annelida have a segmented body structure

Mollusca

• A diverse group of animals that includes the?snails, slugs,?mussels,?limpets, and?octopuses
  • It is the second-largest phylum (i.e. it contains a higher number of recorded species than any other phylum, except for Arthropoda)
• Most molluscs are?aquatic, with?some terrestrial species
• The body structure contains a?muscular foot?and a?visceral?mass
  • The foot is used for?motion?and/or?anchorage?e.g. as in limpets
  • The visceral mass is located above the foot and?contains the internal organs?(i.e. it is the soft, metabolic region that contains the digestive, excretory and reproductive organs)
• The?gut?has separate openings for the?mouth?and?anus
• Mollusca have several feeding methods, including:
  • Filter feeding?e.g. in bivalves such as mussels
  • Scraping at food sources such as leaves or algae on rocks using a rough, tongue-like organ called a?radula?e.g. snails and limpets
  • Active?predation?e.g. sea slugs
• Most mollusca have an?open?circulatory system?in which the transport fluid bathes the internal organs
  • The cephalopods, which include squid and octopuses have a closed circulatory system
• There is?bilateral?symmetry?in the body plan of the?mollusca
• Mollusca have?soft bodies?that sometimes secrete a?calcium carbonate shell from a mantle e.g. snails

The body structure of snails; members of phylum Mollusca

Arthropoda

• Animals with?segmented bodies, a?hard exoskeleton, and?jointed limbs
  • It is the?largest phylum?(i.e. it contains a higher number of recorded species than any other phylum)
  • It includes?insects, arachnids, crustaceans, centipedes, and?millipedes
• There is usually?one pair of jointed limbs per body segment, though this is not always the case
• The?gut?has separate openings for the?mouth?and?anus
• Arthropoda have an?open?circulatory system
• There is?bilateral?symmetry?in the body plan of the?arthropoda
• Arthropoda have a?hard exoskeleton?made of?chitin
  • The exoskeleton is shed as the animal grows

Insects are members of the phylum Arthropoda; they have three body segments, each with a pair of jointed limbs

Chordata

• The phylum?chordata?includes?all of the vertebrates, as well as?some invertebrates?such as the tunicates; commonly known as sea squirts
• To be classified as a chordate, an organism must have the following features?at?some stage during its development:
  • A rod-like structure along the?dorsal?length of the body known as a?notochord
    • Dorsal refers to the?back?of an organism
  • A?hollow nerve cord?located in the?dorsal?region
  • A series of slits behind the mouth known as?pharyngeal slits
  • A?tail?that extends?beyond the anus, known as a?post-anal tail
• Some chordates only have these features during the?embryonic stages?of development, e.g.
  • In vertebrates, the notochord is replaced with the spinal column in fully developed individuals
  • The dorsal hollow nerve cord develops into the brain and spinal cord in most species of chordate
  • The pharyngeal slits are only retained into adulthood in some types of chordate e.g. in the gills of fish
  • The post-anal tail has become a vestigial structure in the great apes
    • Vestigial structures are those that have lost their function, and are sometimes known as 'evolutionary leftovers'

Human embryos have all of the features of phylum chordata

Phyla Features Table

Exam Tip

• The vertebrates are organisms that have a?spinal column?made up of?vertebrae
• There are 5?major classes?of vertebrate
  • Bony ray-finned fish (27 000 species)
    • There are other classes of fish, e.g. the bony lobe-finned fish (6 species), and the cartilaginous fish, whose skeletons are made of cartilage rather than bone (around 900 species)
  • Amphibians (6000 species)
  • Reptiles (9000 species)
  • Birds (10 000 species)
  • Mammals (5 700 species)
• The 5 classes can be recognised by their features

Bony ray-finned fish

• The?ray-finned fish, also known as class Actinopterygii, have?skeletons made of bone, and?fins supported by rays, or spines, of?bone
• The?scales?covering the bodies of the ray-finned fish vary in shape and composition
• Oxygen is gained via?diffusion from the water into the gills, which are covered by a gill flap called the?operculum
• During reproduction, eggs and sperm are released into the water and?fertilisation?takes place outside the body; this is?external fertilisation
• Their entire?life cycle?is?aquatic
• Body temperature is not regulated?but changes with the surrounding environmental temperatures
• Buoyancy is regulated by inflating and deflating an internal air sac called a?swim bladder

Ray-finned fish have a skeleton of bone, fins with bone rays, a swim bladder, and a gill operculum

Amphibians

• This class includes animals such as?frogs, toads, and?salamanders
• Amphibians have?soft, moist skin
• Amphibian skin is?permeable, enabling exchange of gases while under the water
  • Simple lungs are also present for gas exchange on land
• During reproduction, fertilisation is?external?
• Jelly-like eggs?must be laid in water, and the?larval stage of the life-cycle is aquatic?e.g. frog tadpoles
  • Larvae such as tadpoles mature into an adult body form
  • Adult amphibians often leave the water and spend most of their time on land
  • There are a few exceptions to this; some frog and toad species can lay eggs on land which hatch into tiny adults rather than tadpoles
• Body temperature is?not regulated

The amphibian life-cycle includes an aquatic larval stage, and a terrestrial adult stage

Reptiles

• This class includes animals such as?lizards,?snakes, and?turtles
• Reptiles have?impermeable, scaly skin?made of keratin protein
• Reptiles are mainly?terrestrial, gaining their oxygen through highly folded surfaces in their?lungs
• During reproduction, male sperm is deposited inside the female's body, so fertilisation is?internal
• The eggs of reptiles are?soft-shelled
• Reptile body temperature is?not regulated

Reptiles have a body covering of scales, and breathe using highly folded lungs

Birds

• This class includes animals such as?chickens?and?ostriches
• Skin is covered with?feathers?made of keratin
• The front limbs of birds are modified into?wings, and?skeletons are lightweight, both adaptations for?flight
• Birds gain oxygen via a pair of?lungs?and connected?air sacs
• Fertilisation is?internal
• The eggs of birds are?hard-shelled
  • Shells contain calcium carbonate
• Bird?body temperature is?regulated, so is often warmer than the surrounding environment

Birds are usually highly adapted for flight and reproduce using internal fertilisation

Mammals

• This class includes animals such as?elephants,?humans,?and?whales
• Skin is covered with?hair?made of keratin
  • In some mammals body hair is lost in adulthood, e.g. in aquatic mammals
• Mammals gain oxygen via a pair of?lungs
• Fertilisation is?internal
• The young of mammals develops?inside the body?of the mother, and?live young?are born
  • There are some unusual exceptions to this e.g. the duck-billed platypus and the echidna
• Young mammals are fed on?milk?produced in?mammary glands
• Mammal?body temperature is regulated

Vertebrate Features Table