Animals Without a Backbone: Quick Revision

A brief note on Animals Without a Backbone. Read it before a test. Try each question at the end before you open its answer.

The simplest phyla

• Porifera (sponges): cellular level; water in by ostia, through the spongocoel, out by the osculum; collar cells; spicules or spongin; hermaphrodite. Sycon, Spongilla, Euspongia.
• Cnidaria: cnidoblasts with nematocysts; diploblastic; one opening to the gut; polyp and medusa; metagenesis in Obelia. Physalia, Adamsia, Pennatula, Gorgonia, Meandrina, Aurelia.
• Ctenophora (comb jellies): eight rows of comb plates; bioluminescence; only sexual reproduction. Pleurobrachia, Ctenoplana.

The worms

• Platyhelminthes: flat, acoelomate, hooks and suckers, flame cells. Taenia, Fasciola, Planaria.
• Aschelminthes: round, pseudocoelomate, muscular pharynx, sexes separate. Ascaris, Wuchereria, Ancylostoma.
• Annelida: metameres, true coelom, nephridia, closed circulation, double ventral nerve cord; parapodia in Nereis. Pheretima, Hirudinaria.

The higher non-chordates

• Arthropoda: the largest phylum; chitin exoskeleton, jointed appendages, open circulation, malpighian tubules. Apis, Bombyx, Laccifer, Anopheles, Culex, Aedes, Locusta, Limulus.
• Mollusca: head, foot, visceral hump, mantle, radula. Pila, Pinctada, Sepia, Loligo, Octopus.
• Echinodermata: spiny skin of ossicles, water vascular system, radial adult and bilateral larva, no excretory system. Asterias, Echinus, Antedon, Cucumaria, Ophiura.
• Hemichordata: proboscis, collar and trunk; proboscis gland. Balanoglossus, Saccoglossus.

A worked example

A tray from a field trip holds $13$ insects, $4$ spiders and $3$ prawns. How many legs are on the tray?

1. Insects. Three pairs each: $13 \times 6 = 78$ legs.
2. Spiders. Four pairs each: $4 \times 8 = 32$ legs.
3. Prawns. Five pairs each: $3 \times 10 = 30$ legs.
4. All the legs. $78 + 32 + 30 = 140$ legs.

All of them are arthropods: their legs are jointed.

Practice with new numbers

A program to try

The program names the phylum of three animals from one telling feature each.

Mistakes to avoid

• A jellyfish, a starfish and a cuttlefish are not fish. None has a backbone.
• Water enters a sponge by the ostia, not by the osculum.
• Roundworms are pseudocoelomate, not coelomate.
• Earthworms and leeches are monoecious; Nereis is dioecious.
• Echinoderm larvae are bilateral; only the adults are radial.

Check yourself

1. Which phylum has a gastro-vascular cavity with only one opening?

2. Name the organs of excretion in flatworms, annelids and insects.

3. Which mollusc is the pearl oyster?

4. Give the three uses of the water vascular system.

5. Why is Balanoglossus no longer placed among the chordates?