Animal Phyla Features (DP IB Biology: SL)

• 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

• 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