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Must-know facts
20 facts
- 1Cellular level: Porifera. Tissue level: Coelenterata and Ctenophora. Organ level: Platyhelminthes. Organ-system level: Aschelminthes onwards.
- 2Incomplete gut (one opening): Platyhelminthes. Open circulation: arthropods and hemichordates; closed: annelids and chordates.
- 3Radial symmetry: coelenterates, ctenophores, adult echinoderms. Echinoderm larvae are bilateral. Sponges are mostly asymmetrical.
- 4Diploblastic with mesoglea: Coelenterata (and Ctenophora). Triploblastic: Platyhelminthes to Chordata.
- 5Acoelomate: Platyhelminthes. Pseudocoelomate: Aschelminthes. Coelomate: annelids onwards, including hemichordates and chordates.
- 6Metamerism is first seen in Annelida (earthworm).
- 7Water path in sponges: ostia → spongocoel → osculum; choanocytes line the canals; digestion intracellular.
- 8Cnidoblasts with nematocysts: Coelenterata. Metagenesis: Obelia. Coral skeleton: calcium carbonate.
- 9Ctenophores: eight rows of comb plates, bioluminescence, only sexual reproduction.
- 10Flame cells: Platyhelminthes. Nephridia: Annelida. Malpighian tubules: Arthropoda. Proboscis gland: Hemichordata. No excretory system: Echinodermata.
- 11Parapodia: Nereis (dioecious). Earthworm and leech: monoecious.
- 12Arthropoda: largest phylum, over two-thirds of named species; Limulus is a living fossil.
- 13Mollusca: second largest phylum; radula for feeding; gills in the mantle cavity.
- 14Water vascular system: Echinodermata. Stomochord: Hemichordata (now a non-chordate phylum).
- 15Notochord only in the larval tail: Urochordata. Head to tail and lifelong: Cephalochordata.
- 16Cyclostomes: 6-15 pairs of gill slits, jawless sucking mouth, ectoparasites on fishes.
- 17Chondrichthyes: placoid scales, no air bladder, no operculum, claspers in males. Osteichthyes: four pairs of gills under an operculum, air bladder.
- 18Heart: fishes 2 chambers; amphibians and most reptiles 3; crocodiles, birds and mammals 4.
- 19Poikilotherms: fishes, amphibians, reptiles. Homoiotherms: birds and mammals.
- 20Birds: pneumatic bones, crop and gizzard, air sacs, only an oil gland at the tail base. Platypus: an egg-laying mammal.
Common traps
Where marks are lost
Calling echinoderms bilaterally symmetrical, or radially symmetrical at every stage.
The adult is radial, the larva is bilateral. The chapter flags Echinodermata as radial or bilateral depending on the stage.
Placing Hemichordata inside Chordata because it has a notochord.
It has only a stomochord, similar to a notochord, in the collar. It is now a separate non-chordate phylum.
Saying all reptiles have a three-chambered heart.
Usually three chambers, but crocodiles have four.
Saying all annelids are hermaphrodite.
Earthworms and leeches are monoecious, but Nereis is dioecious.
Writing that aschelminthes are acoelomate or coelomate.
They are pseudocoelomate: the cavity is not lined by mesoderm. Flatworms are the acoelomates.
Assuming all mammals give birth to live young.
Viviparous with few exceptions; Ornithorhynchus (platypus) lays eggs.
Mixing up the protochordates: putting the lifelong notochord in Urochordata.
Urochordata: notochord only in the larval tail. Cephalochordata (Branchiostoma): head to tail, throughout life.
Giving cartilaginous fishes an air bladder or an operculum.
Chondrichthyes lack both, so they swim constantly to avoid sinking. Bony fishes have both.
Thinking sponges have separate sexes or external fertilisation.
Sponges are hermaphrodite with internal fertilisation and indirect development.
Key terms
20 terms
- Choanocyte
- A collar cell lining the spongocoel and canals of a sponge.
- Spongocoel
- The central cavity of a sponge; water leaves it through the osculum.
- Cnidoblast
- A cnidarian cell carrying a stinging capsule, the nematocyst.
- Metagenesis
- Alternation of polyp and medusa forms in a cnidarian such as Obelia.
- Mesoglea
- The undifferentiated layer between ectoderm and endoderm in diploblastic animals.
- Coelom
- A body cavity lined by mesoderm, between body wall and gut wall.
- Pseudocoelom
- A body cavity not lined by mesoderm; the mesoderm lies as scattered pouches.
- Metamerism
- Division of the body into serially repeated segments, outside and inside.
- Notochord
- A dorsal, mesoderm-derived rod formed in the embryo of chordates.
- Flame cells
- Excretory and osmoregulatory cells of flatworms.
- Nephridia
- Excretory and osmoregulatory organs of annelids.
- Parapodia
- Lateral swimming appendages of aquatic annelids such as Nereis.
- Malpighian tubules
- Excretory organs of arthropods.
- Radula
- The file-like rasping feeding organ in a mollusc's mouth.
- Water vascular system
- The echinoderm system used for locomotion, food capture and transport, and respiration.
- Stomochord
- A rudimentary notochord-like structure in the collar of hemichordates.
- Placoid scales
- Minute scales in the skin of cartilaginous fishes; their teeth are modified placoid scales.
- Cloaca
- A common chamber into which the gut, urinary and reproductive tracts open.
- Pneumatic bones
- Hollow long bones of birds with air cavities.
- Homoiothermous
- Able to keep a constant body temperature; birds and mammals.