21. A biologist places a single, living sponge in a saltwater tank. After several weeks, the biologist observes other small sponges living in the tank. Because the biologist is certain that no other sponge was introduced into the tank, what other explanation could you provide to explain the observation?
22. Describe the body of a leech. In what way do the leech’s adaptations make it suited for its niche?
23. Segmented tube worms that live near the vents grow to lengths of 1.5 m in contrast to the growth of related tube worms living in other environments, whose growth is measured only in centimeters at most. You hypothesize that the food that the worms eat is more abundant at the vents. When you collect samples of the worms, you discover that they have no mouth or other means of taking in food. Hypothesize how the tube worms are obtaining nutrients.