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Across 1. embryo stays inside mother's uterus 7. protects developing embryo 10. balloon like organ that is filled with oxygen and other gases from the bloodstream that keep the fish from sinking 16. time when an embryo develops within the mother 18. the bones in the segmented columns 23. animals with a backbone 24. ancestors of mammals 25. organs that remove oxygen from the water 26. protect fishes body and reduce friction as they swim through the water

Down 2. made of a stiff central shaft with many side branches 3. saclike organ that takes oxygen from the air and delivers it to the blood 4. row of tiny sense organs along each side of the fishes body that extend to the head 5. fanlike structures that help fish move, steer, stop, and balance 6. secrete milk 8. mammals that lay eggs 9. mammals with pouches 11. when birds sit on their eggs using their body heat to keep the eggs warm until they hatch 12. the change from larva form to adult form 13. animals that do not control their body temperature through the chemical reactions of their cells 14. animals that maintain a constant body tempertaure 15. fluffy, insulating feathers that lie next to a bird's body 17. muscle that helps bring air to lungs 19. cover the skin of cartilaginous fish 20. upward force on the wing that keeps a bird in the air 21. when birds use their beaks to spread oil on the feathers to clean them 22. the amphibian embryo developing as a aquatic larva