Arthropods, molluscs, echinoderms and hemichordates

The biggest phyla

More than two thirds of all the named kinds of animals on Earth are arthropods. The next largest group is the molluscs. In this lesson you meet both, and then two groups of sea animals: the spiny echinoderms and the worm-like hemichordates.

Phylum Arthropoda: jointed legs

This is the largest phylum of animals, and it includes the insects. The name means "jointed feet" (arthros, joint; poda, feet).

• They have an organ-system level of organisation, and are bilaterally symmetrical, triploblastic, segmented and coelomate.
• The body is covered by a hard exoskeleton made of chitin.
• The body has a head, a thorax and an abdomen.
• The legs and other appendages are jointed.
• They breathe with gills, book gills, book lungs or a tracheal system.
• The circulatory system is open: the blood flows through open spaces in the body.
• Sense organs include antennae, eyes (compound and simple) and statocysts, which are organs of balance.
• Malpighian tubules remove wastes.
• Most are dioecious, and fertilisation is usually internal. Most lay eggs: they are oviparous. Development may be direct or indirect.

Examples worth knowing.

Insects have six legs, spiders eight, and crabs and prawns ten. All the legs are jointed.

Phylum Mollusca: soft bodies

This is the second largest animal phylum. Molluscs live on land or in water, in the sea or in fresh water.

• They have an organ-system level of organisation, and are bilaterally symmetrical, triploblastic and coelomate.
• The body is soft and unsegmented, usually covered by a calcareous shell.
• It has a distinct head, a muscular foot and a visceral hump.
• A soft, spongy layer of skin, the mantle, covers the visceral hump. The space between them is the mantle cavity. Feather-like gills in it serve for breathing and excretion.
• The head has sensory tentacles.
• The mouth holds a file-like rasping organ for feeding, the radula.
• They are usually dioecious and oviparous, with indirect development.

Examples: Pila (the apple snail), Pinctada (the pearl oyster), Sepia (the cuttlefish), Loligo (the squid), Octopus (the devil fish), Aplysia (the sea hare), Dentalium (the tusk shell) and Chaetopleura (the chiton).

Some of these are called fish, but none of them is a fish. A cuttlefish has no backbone; it is a mollusc.

Phylum Echinodermata: spiny skins

The name means "spiny skin". Their endoskeleton is built of hard calcareous ossicles.

• All echinoderms are marine, with an organ-system level of organisation.
• The adult is radially symmetrical, but the larva is bilaterally symmetrical.
• They are triploblastic and coelomate.
• The digestive system is complete. The mouth is on the lower side, and the anus on the upper side.
• Their most distinctive feature is the water vascular system. It helps in movement, in catching and passing food, and in breathing.
• An excretory system is absent.
• The sexes are separate. Fertilisation is usually external, and development is indirect, with a free-swimming larva.

Examples: Asterias (the starfish), Echinus (the sea urchin), Antedon (the sea lily), Cucumaria (the sea cucumber) and Ophiura (the brittle star).

Phylum Hemichordata

Hemichordates were once placed as a subphylum under Chordata. They are now a separate phylum among the non-chordates.

• They are a small group of worm-like marine animals, with an organ-system level of organisation.
• They are bilaterally symmetrical, triploblastic and coelomate.
• The body is cylindrical, with three parts: an anterior proboscis, a collar and a long trunk.
• The circulatory system is open. They breathe through gills.
• The excretory organ is the proboscis gland.
• The sexes are separate. Fertilisation is external, and development is indirect.

Examples: Balanoglossus and Saccoglossus.

You should find that $12$ insects, $5$ spiders and $2$ crabs carry $72$, $40$ and $20$ legs, $132$ legs in all.

Try it in Python

The program counts the legs in a jar of arthropods, kind by kind.

You should see $132$ legs in all. A centipede with $30$ legs brings the total to $162$.

Practise with new numbers

Common mistakes

• Calling a starfish a fish. Asterias is an echinoderm. It has no backbone.
• Saying that echinoderms are radial all their life. Only the adult is radial; the larva is bilateral.
• Calling a spider an insect. Both are arthropods, but a spider has eight legs and no antennae.
• **Putting Balanoglossus among the chordates.** It is now placed in its own non-chordate phylum, Hemichordata.

Remember these

• Arthropoda: the largest phylum; chitin exoskeleton, jointed appendages, open circulation, malpighian tubules.
• Mollusca: head, foot and visceral hump; mantle; radula; usually a calcareous shell.
• Echinodermata: spiny skin of ossicles, water vascular system, radial adult and bilateral larva, no excretory system.
• Hemichordata: proboscis, collar and trunk; proboscis gland; Balanoglossus.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name three insects that carry disease, and three that are useful to people.

2. What is the radula, and in which phylum is it found?

3. A starfish larva swims freely and has a left side and a right side. Is the adult the same? Explain.

4. Which system of an echinoderm helps it to move, to feed and to breathe?

5. Name the three parts of the body of Balanoglossus.