Biology  5/e   Raven/Johnson
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Eukaryotic cell structure: All organisms are composed of one or more cells, and all cells are either prokaryotic or eukaryotic. Bacteria are composed of prokaryotic cells; all other organisms are eukaryotic. Eukaryotic cells are more complex than prokaryotic cells and have a highly organized interior with many different functional compartments. Organelles found inside eukaryotic cells, but not prokaryotic cells, include a membrane-bound nucleus, endoplasmic reticulum, Golgi bodies, lysosomes, microbodies, mitochondria, chloroplasts, and multiple chromosomes. Like prokaryotic cells, eukaryotic cells have a plasma membrane and sometimes a cell wall.

Evolution of vertebrates: Vertebrates represent a major subphylum of the chordates. Chordates and echinoderms represent the two major phyla of deuterostomes, which evolved about 630 million years ago and have distinct embryological development patterns. The first vertebrates to appear were the jawless fish. They evolved at least 470 million years ago and approximately 410 million years ago gave rise to the jawed fish. Lobe-finned fishes gave rise to the amphibians about 350 million years ago. Approximately 300 million years ago the amphibians gave rise to the reptiles. One branch of reptiles gave rise to birds 225 to 150 million years ago, and another branch of reptiles gave rise to mammals about 200 million years ago. All vertebrates are characterized by a vertebral column surrounding a dorsal nerve cord.

Basic structure of chordates: Chordates are animals characterized by three principal features: (1) a hollow nerve cord running just beneath the dorsal surface, (2) a notochord, a flexible rod running parallel to and between the nerve cord and the gut, and (3) pharyngeal slits. The notochord and pharyngeal slits may be present only during certain embryological stages or may persist throughout the life of the chordate, depending on the species. Chordates also tend to have bilateral symmetry, some degree of body segmentation, distinct blocks of muscles, an internal skeleton, and a tail that extends beyond the anus.

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