A child finds a starfish ( Asterias ) in a rock pool and calls it a fish…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A child finds a starfish (Asterias) in a rock pool and calls it a fish. (a) Give two reasons why it is not a fish. (b) Name its phylum, and the system of water-filled canals that works its tube feet. (c) Its larva swims with a left and a right side. What does this tell you about its symmetry? (d) Why do biologists not place Balanoglossus, another sea animal, in this phylum?
Need a reminder? The key ideaShow
Arthropoda: the largest phylum; chitin exoskeleton, jointed appendages, head, thorax and abdomen, open circulation, malpighian tubules; insects have 6 legs, spiders 8, crabs 10. Mollusca: head, muscular foot, visceral hump, mantle, radula, usually a shell; Pila, Pinctada, Sepia, Loligo, Octopus. Echinodermata: calcareous 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.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Not a fish. It has no backbone and no fins. Its skeleton is made of calcareous ossicles, not bone.
Step 2. The phylum. It is an echinoderm. The water vascular system works its tube feet, for moving, feeding and breathing.
Step 3. Symmetry. The larva is bilateral, but the adult is radial. The five-armed plan comes later in life.
**Step 4. Balanoglossus. It is worm-like, with a proboscis, a collar and a trunk, and no water vascular system. It belongs to Hemichordata**.
Answer: (a) no backbone, no fins (a skeleton of ossicles); (b) Echinodermata, the water vascular system; (c) bilateral larva, radial adult; (d) it has a proboscis, collar and trunk and no water vascular system, so it is a hemichordate.
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