1						2					3		4						5
		6																7
										8
												9
			10																											11		12
																	13
	14
						15
													16			17										18
19																								20
									21																			22
				23																				24
																25			26									27
				28					29
																																	30
								31
										32
																															33
		34
		35																									36
37																						38
														39
										40																			41
				42
												43					44								45
46											47												48
																					49
								50																	51
52																			53

Across 5. Organs that are found in some adult amphibians, reptiles, birds, and mammals, which help with acquiring oxygen. 6. An ectothermic vertebrate that has lungs and scaly skin. 7. A phylum of animals with jointed legs and exoskeletons. 9. The first bird, which had several bird-like and several reptile-like characteristics. 10. An organism that generates body heat to maintain a constant body temperature. 13. Feathers that help a bird hold in heat. 17. An animal that has a notochord, slits in its throat, and a nerve cord. 19. Feathers that give a bird's body shape to support flight. 20. A fish with a skeleton made of bone. 21. A phylum of animals that includes round worms. 22. A large tooth with a flat surface used for grinding food. 23. A liquid used as nourishment for mammal young. 29. An ectothermic vertebrate that lives in the water for part of its life and on land for the other part. 32. The type of heart that you would find in a fish. 34. The type of heart that you would find in a bird or mammal. 37. A fish with no scales, paired fins, or jaw. 38. A sense organ used by some fish to detect movement in the water. 40. A phylum of animals that includes sea stars, sea urchins, and sea cucumbers. 42. A layer on mammals made of the same material as feathers that provides insulation to hold in heat. 46. Organs that are found in fish and young amphibians which help with acquiring oxygen. 47. A group of mammals that give birth to live young that are at a very early stage of development. 48. A phylum of animals that includes jellyfish and sea anemones. 49. A group of mammals that lay eggs. 50. The only type of marsupial found naturally in the United States. 51. A class of animals which provide milk for their offspring. 52. A heart chamber that receives blood. 53. Structures on the skin of reptiles that help hold in water.

Down 1. Structures found on birds that provide shape to the body and help hold in heat. 2. An organ that acts as a filter and a barrier between the mother and baby, through which food, oxygen, and water can pass. 3. A digestive system organ found in birds used for storing food. 4. The type of heart that you would find in an amphibian. 5. A parasitic jawless fish. 8. An animal that has a backbone. 11. A heart chamber that sends blood out of the heart. 12. A flexible rod-like structure that provides support. 14. An internal skeleton. 15. A fish with a jaw and a skeleton made of cartilage. 16. The time between fertilization and birth of offspring, during which the offspring is developing. 18. An animal without a backbone. 24. A phylum of animals that includes flatworms. 25. Organs found in mammals which produce milk. 26. Mammals that give birth to relatively well-developed young after longer periods of gestation. 27. An animal whose body does not produce much internal heat. 28. An egg-laying mammal that swims in the water and has a bill similar to a duck. 30. The bones that make up a backbone. 31. Archaeopteryx had these in its mouth, which was more reptilian. 33. A phylum of animals that includes segmented worms. 35. An internal gas-filled sac that bony fish have to rise or sink in the water. 36. A flexible material used for providing structure, such as in your nose or ear lobes. 39. A flat-edged tooth used for cutting and gnawing. 41. A phylum of animals that includes sponges. 42. A vertebrate with fins that lives in the water. 43. A scavenging jawless fish known for producing slime. 44. A digestive system organ found in birds which contains pebbles to help grind up food. 45. The upward force for bird flight. 48. A sharp, pointed tooth used for ripping and tearing flesh.