Animal Phyla: Sponges, Cnidarians, Molluscs, Arthropods and Vertebrates

Sponges (Porifera)

Sponges are asymmetrical or radially symmetrical animals with a body wall that surrounds an atrial cavity. Their bodies are covered in pinacocytes and their water canals are lined with choanocytes, which create a current of water through the body. The intermediate layer, the mesohyl, contains amoebocytes, which digest food particles, transport nutrients and manufacture spicules. Sponges reproduce both sexually and asexually.

Types of sponges

• Calcareous sponges: with calcium carbonate spicules.
• Siliceous sponges: with silica spicules.
• Demosponges: the most common sponge group.

Cnidarians

Cnidarians occur as polyps and medusae. They have cnidoblasts in the epidermis containing stinging cells (cnidocytes). Amoeboid cells in the mesoglea and the gastrodermis with pseudopods help capture food. They also have a nerve net.

Molluscs

Gastropods

Gastropods are terrestrial, marine and freshwater animals with asymmetrical bodies, a spiral shell, a large muscular foot for movement, sensory tentacles with eyes at the base, a radula, and gills or lungs. Their external or internal shell may be present or absent. Examples include snails, slugs, limpets and sea hares.

Bivalves

Bivalves live in the sea or in rivers. They have bilateral symmetry, a shell of two valves, and a compressed foot for burrowing. They lack a head, tentacles and a radula, and breathe through gills. Examples include mussels, oysters, clams and scallops.

Cephalopods

Cephalopods are marine molluscs with large heads, bilateral symmetry, an internal or absent shell, and a foot modified into tentacles. Their head is highly developed and they have gills and an internal fertilisation system. Examples include squid, octopus, cuttlefish and nautiluses.

Annelids

Annelids have a body divided into segments (metamerism) internally and externally. Groups include:

• Polychaetes: marine worms.
• Oligochaetes: earthworms.
• Hirudinea: leeches.

Arthropods

Arthropods share these characteristics:

1. An exoskeleton made of chitin that is shed during moulting.
2. A respiratory system with tracheae or gills.
3. Sensory organs that are well developed.
4. Complex behaviour.
5. A decrease in intraspecific competence during metamorphosis.

The body is divided into three regions: head, thorax and abdomen. Chelicerates have a cephalothorax and an abdomen without segmentation, with appendages arranged in pairs. Mandibulates have jaws on the head and include myriapods, insects and crustaceans.

Types of arthropods

• Arachnids: three body regions, bilateral symmetry, a cephalothorax and abdomen without antennae, four pairs of legs, tracheae or book lungs, no metamorphosis. Examples: spiders, scorpions.
• Crustaceans: aquatic, bilateral, four pairs of antennae, mouthparts for chewing, and variable leg numbers. Examples: crabs, lobsters.
• Insects: three body regions (head, thorax, abdomen), two antennae, mouthparts, three pairs of legs, tracheae, and often two pairs of wings. Examples: butterflies, wasps.
• Myriapods: terrestrial, bilateral, two antennae, two jaws, many legs, tracheae. Examples: centipedes, millipedes.

Echinoderms

Echinoderms have a water vascular system with ambulacral tube feet that allow them to move. The ambulacral system opens to the exterior through a madreporite plate. Types include asteroids (starfish from the sea) and echinoids (sea urchins).

Chordates

Chordates have a notochord at some stage of their life cycle, a single dorsal tubular nerve cord, pharyngeal slits, and a post-anal tail at some stage of development.

Vertebrates

Vertebrates have an endoskeleton with vertebrae, a well-developed nervous system, pharyngeal slits, a closed circulatory system, and kidneys that filter internal liquids. They reproduce sexually.

Gnathostomes

Chondrichthyes (cartilaginous fish)

Chondrichthyes have skin covered with placoid scales and teeth, a cartilaginous skeleton, asymmetrical pectoral fins, no swim bladder, five pairs of gill slits, internal fertilisation, and are poikilothermic. Examples: sharks, rays.

Osteichthyes (bony fish)

Osteichthyes have skin covered with scales, a bony skeleton, pectoral and pelvic fins, a symmetrical tail fin, a swim bladder, gill slits protected by a bony operculum, external fertilisation by eggs, and are poikilothermic. Examples: trout, salmon.

Amphibians

Amphibians have skin without scales that is also used for breathing, a bony skeleton, and legs. Tadpoles have gill slits, while adults have lungs. They have internal fertilisation in water and are poikilothermic. Examples: frogs, toads.

Reptiles

Reptiles have dry skin covered with scales, a bony skeleton, legs, lungs, internal fertilisation, and eggs, and are poikilothermic. Examples: lizards, snakes, turtles, crocodiles.

Birds

Birds have feathers, a bony skeleton, legs, lungs, and eggs with a calcareous shell. They are homeothermic. Examples: eagles, penguins, sparrows.