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List three characteristics that distinguish cartilaginous fishes from living jawless fishes.

Identify the advantages of internal fertiization versus external fertilization.

7. Evaluating Differences Organisms that use external fertilization usually produce more eggs at one time than organisms that use internal fertilization. What might explain this difference?

8. Inferring Relationships Explain why all scales point toward a fish's tail.

Modeling a Shark Adaptation

Materials 8 cm dialysis tubing, 8 cm length of string (2), 100 mL salt solution (5 percent), 250 mL beaker, 10 mL distilled water, scale, graduated cylinder

Procedure

1. Tightly tie one end of the dialysis tubing with string. Place 10 mL of distilled water inside the dialysis tubing, and tie the other end of the tube tightly with string.

2. Record the initial weight of the filled tube.

3. Add the filled dialysis tubing to a beaker filled with 100 mL of salt solution.

4. After 10 minutes, remove the tubing from the beaker, blot the outside, and reweigh it. Record your observations.

Analysis Explain the reason for the change in the weight of the tube, if any. Are sharks likely to lose or gain ions? What physical structures of a shark play a key role in maintaining ionic homeostasis in sharks?

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