The class of vertebrates that includes the fish that have skeletons consisting of firm rubber-like cartilage rather than bone. These are the sharks and rays.

The class of vertebrates that includes the jawless fish that do not have scales. These are the lampreys and hagfish.

The class of vertebrates that include the fish that have skeletons consisting of bone. Most fish species are in this class (e.g., tuna, bass, perch, salmon, trout, etc.).

A kind of egg which usually has a hard shell that protects the embryo from drying out. This is an advantage because it can be laid on land where it is usually safer from predators than it would be in lakes, rivers, and oceans. These eggs are produced by reptiles, birds, and some primitive mammals.

The class of vertebrates that includes turtles, snakes, lizards, alligators, and dinosaurs. They all have lungs to breathe on land and skin that does not need to be kept wet.

The vertebrate class that includes animals that spend part of their lives under water and part on land. All of these animals must return to damp areas to reproduce since their eggs would dry out otherwise. They start life with gills, like fish, and later develop lungs to breath air.

The vertebrate class that includes animals that have wings and can produce amniote eggs. In the case of both reptiles and these creatures, the eggs are fertilized within the reproductive tract of females.

The class of warm blooded, usually hairy animals, that feed their young with milk produced by mothers. This class includes monotremes, marsupials, primates, cats, dogs, bears, hoofed animals, rodents, bats, seals, dolphins, and whales.

The milk secreting glands of mammals. They are usually located on the chest or abdomen in one or more bilaterally symmetrical pairs. They are also referred to as breasts.

The term referring to the characteristic of having a variety of specialized teeth. Mammals have this characteristic but the other classes of vertebrates do not.

The ability of reptiles, amphibians, fish, and insects to maintain their core body temperature in a normal range mainly by avoiding exposure to environmental temperature extremes. This is also referred to as being cold blooded.

The ability of birds and mammals to maintain a relatively constant core body temperature regardless of external conditions by using internal physiological means. That is to say, they are homeothermic or stable in core body temperature. This is also referred to as being warm blooded.

The subclass of mammals that include the marsupials and placental mammals. They have in common the fact that they give birth to live young instead of laying eggs.

The subclass of mammal species in which females lay eggs like non-mammalian vertebrates. However, they feed their newborn with mammary gland secretions like all other mammals. They are also referred to as monotremes, which literally means that they have one opening for excretion and reproduction. Included in this subclass are the platypus and echidna.

The infraclass of therian mammal species in which females give birth to their young in an immature condition (while still in the early fetal stage) and then permit their further infant development in an abdominal pouch covering their mammary glands. They are also called marsupials. Included in this infraclass are kangaroos, koalas, and opossums.

The infraclass of therian mammal species in which females produce a placenta to connect the fetus to the uterus. This enables nutrients and oxygen to get to the fetus and provides a means of eliminating waste products. As a result, these mammals can carry their young within the uterus until late in fetal development. This has a selective advantage because it results in decreased infant mortality. They are also called placental mammals. Included in this infraclass are dogs, cats, bears, whales, and primates.

The mammal subclass and infraclass in which humans are classified.