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Your guide: Miss AimanThinks the human body is the best-designed machine in the syllabus.
An earthworm, a snail and a sea star are all animals. How are they different?
Let us sort animals by body layers, cavity and body parts.
We walk through the phyla one by one.
Animals are multicellular eukaryotes. They are heterotrophs. They develop from embryos. They ingest food and digest it inside the body.
Porifera are sponges. No tissues. No nervous system. The body has pores called ostia (in) and a larger osculum (out).
Sponge layers: pinacocytes outside, mesohyle in the middle, choanocytes (collar cells) lining the spongocoel.
Sponges reproduce by budding or regeneration. Gemmules are resistant capsules.
Cnidaria: radial symmetry, diploblastic, mesoglea between the layers. Cnidocytes hold a nematocyst for defence and catching prey.
They have a gastrovascular cavity with one opening. There is no central nervous system. Two body forms: polyps (attached, asexual) and medusae (free-swimming, sexual).
Platyhelminthes: flat bodies, triploblastic, acoelomate, bilateral symmetry. Free-living: planaria. Parasites: liver fluke, tapeworm.
Flame cells and protonephridia handle excretion. They are hermaphrodites.
Nematoda: roundworms. Pseudocoelomates with a fluid-filled false body cavity. Complete gut with a mouth and an anus. Unisexual. Examples: Ascaris, hookworm, pinworm.
Mollusca: soft unsegmented body, true coelom. Three parts: head, visceral mass, foot. A mantle often makes a shell. Classes: Gastropoda (snail), Bivalvia (mussel, oyster), Cephalopoda (octopus, cuttlefish).
Annelida: segmented worms. Metameric segmentation, septa, setae (not in leeches). Closed circulation. Haemoglobin in the plasma. Examples: Nereis, leech, earthworm.
Arthropoda: jointed appendages, chitin exoskeleton shed by ecdysis, open circulation with haemolymph, trachea with spiracles, Malpighian tubules, compound eyes. Classes in the book: Crustacea, Insecta, Arachnida, Myriapoda.
Echinodermata: exclusively marine. Adults have radial symmetry, larvae bilateral. Parts come in fives. Calcareous endoskeleton of ossicles. Water-vascular system: ring canal, radial canals, tube feet, madreporite.
Chordates are bilaterally symmetrical, triploblastic, deuterostome coelomates.
Four unique features: notochord, pharyngeal slits, tubular nerve cord, post-anal tail.
Invertebrate chordates: Urochordata (sea squirts) and Cephalochordata (amphioxus).
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An earthworm, a snail and a sea star are all animals. How are they different?
Let us sort animals by body layers, cavity and body parts.
We walk through the phyla one by one.
Animals are multicellular eukaryotes. They are heterotrophs. They develop from embryos. They ingest food and digest it inside the body.
Porifera are sponges. No tissues. No nervous system. The body has pores called ostia (in) and a larger osculum (out).
Sponge layers: pinacocytes outside, mesohyle in the middle, choanocytes (collar cells) lining the spongocoel.
Sponges reproduce by budding or regeneration. Gemmules are resistant capsules.
Cnidaria: radial symmetry, diploblastic, mesoglea between the layers. Cnidocytes hold a nematocyst for defence and catching prey.
They have a gastrovascular cavity with one opening. There is no central nervous system. Two body forms: polyps (attached, asexual) and medusae (free-swimming, sexual).
Platyhelminthes: flat bodies, triploblastic, acoelomate, bilateral symmetry. Free-living: planaria. Parasites: liver fluke, tapeworm.
Flame cells and protonephridia handle excretion. They are hermaphrodites.
Nematoda: roundworms. Pseudocoelomates with a fluid-filled false body cavity. Complete gut with a mouth and an anus. Unisexual. Examples: Ascaris, hookworm, pinworm.
Mollusca: soft unsegmented body, true coelom. Three parts: head, visceral mass, foot. A mantle often makes a shell. Classes: Gastropoda (snail), Bivalvia (mussel, oyster), Cephalopoda (octopus, cuttlefish).
Annelida: segmented worms. Metameric segmentation, septa, setae (not in leeches). Closed circulation. Haemoglobin in the plasma. Examples: Nereis, leech, earthworm.
Arthropoda: jointed appendages, chitin exoskeleton shed by ecdysis, open circulation with haemolymph, trachea with spiracles, Malpighian tubules, compound eyes. Classes in the book: Crustacea, Insecta, Arachnida, Myriapoda.
Echinodermata: exclusively marine. Adults have radial symmetry, larvae bilateral. Parts come in fives. Calcareous endoskeleton of ossicles. Water-vascular system: ring canal, radial canals, tube feet, madreporite.
Chordates are bilaterally symmetrical, triploblastic, deuterostome coelomates.
Four unique features: notochord, pharyngeal slits, tubular nerve cord, post-anal tail.
Invertebrate chordates: Urochordata (sea squirts) and Cephalochordata (amphioxus).
Q1. Why is a sponge not a tissue-level animal?
The book says Porifera have no tissue-level organisation and no nervous system.
✗ “Tapeworms are unsegmented.”
✓ The book's side note says tapeworms in this phylum are segmented. Most flatworms are unsegmented.
1. Which phylum has cnidocytes with nematocysts?
(a) They are for defence and catching prey.
2. Roundworms are:
(a) The fluid-filled cavity is a pseudocoelom.
3. The body of a mollusc has which three parts?
(a) Head, visceral mass and foot.
4. Shedding the exoskeleton in arthropods is called:
(a) It allows growth. Also called moulting.
5. Which is NOT one of the four chordate features in the book?
(a) A chitin exoskeleton belongs to arthropods.
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