3. The hemichordates may bridge echinoderms and chordates.
25.2 The Tunicates and Lancelets Are "Protochordates"
4. The tunicates and lancelets are the only invertebrate chordates.
5. Tunicates have a covering that contains cellulose, and retain the notochord only in the larval tail. They obtain food and oxygen with a siphon system.
6. Lancelets resemble eyeless fishes. They have rudimentary circulatory and nervous systems.
25.3 Vertebrates
7. Vertebrates are distinguished from other animals by a backbone, paired limbs, an endoskeleton, and organ systems.
8. Compared with invertebrate chordates, vertebrates have a diminished notochord, a central nervous system, and in the tetrapods, lungs and limbs.
9. Vertebrates are distinguished from each other by habitat, mode of locomotion, and how they have young. Reptiles, mammals, and birds are amniotes, possessing membranes that protect the fetus.
10. Vertebrates include four fish classes, amphibians, reptiles, birds, and mammals.
11. Fishes are abundant, diverse, and include four classes.
12. Aquatic adaptations include a streamlined body, lightweight bones, fins and tails, and ways to maximize extraction of oxygen from water. They are ectotherms, with the environment controlling body temperature.
13. Hagfishes and lampreys are two types of jawless fishes. The hagfishes are more primitive, and secrete slime. Lampreys have a sucking mouth.
14. Ostracoderms were jawless fishes, now extinct. The placoderms are an extinct class that may have originated jaws.
15. The cartilaginous jawed fishes include chimaeras, skates, rays, and sharks. Sharks detect vibrations from prey with a lateral line system.
16. The bony jawed fishes account for 96% of extant fish species. They include ray-finned fishes, lungfishes, and lobe-finned fishes. Bony fishes have lateral line systems and also swim bladders, which enable them to control their buoyancy.
17. Amphibians retain some characteristics of fishes, yet are adapted to life on land. They have denser bones, lungs, four limbs, different senses emphasized, and more powerful circulatory systems. Chromatophores enable some of them to change color.
18. Amphibians include frogs and toads (order Anura), salamanders and newts (order Urodela), and caecilians (order Gymnophiona).
19. Amphibians have many adaptations to avoid predators, reproduce, and respire in a variety of ways.
25.6 Reptiles Fully Conquer Land
20. Reptilian adaptations to life on land include efficient excretory systems, amniote eggs, better respiration and circulation, internal fertilization, and a cerebral cortex.
26. Defining characteristics include fur, milk secretion, non-nucleated red blood cells, two sets of teeth, oil and sweat glands, and a highly developed cerebral cortex.
27. Primates have excellent vision and coordination, a large brain, five digits per limb, and opposable thumbs.
