Term | Definition |
homeostasis | the tendency toward a relatively stable equilibrium between interdependent elements, especially as maintained by physiological processes. |
adaptation | the action or process of adapting or being adapted. |
vertebrate | an animal of a large group distinguished by the possession of a backbone or spinal column, including mammals, birds, reptiles, amphibians, and fishes. |
invertebrate | an animal lacking a backbone, such as an arthropod, mollusk, annelid, coelenterate, etc. The invertebrates constitute an artificial division of the animal kingdom, comprising 95 percent of animal species and about 30 different phyla. |
tissue | any of the distinct types of material of which animals or plants are made, consisting of specialized cells and their products. |
organ | a part of an organism that is typically self-contained and has a specific vital function, such as the heart or liver in humans. |
radial symmetry | symmetry around a central axis, as in a starfish or a tulip flower. |
bilateral symmetry | the property of being divisible into symmetrical halves on either side of a unique plane. |
cnidarian | an aquatic invertebrate animal of the phylum Cnidaria, which comprises the coelenterates. |
mollusk | an invertebrate of a large phylum that includes snails, slugs, mussels, and octopuses. They have a soft, unsegmented body and live in aquatic or damp habitats, and most kinds have an external calcareous shell. |
arthropod | an invertebrate animal of the large phylum Arthropoda, such as an insect, spider, or crustacean. |
exoskeleton | a rigid external covering for the body in some invertebrate animals, especially arthropods, providing both support and protection. |
echinoderm | a marine invertebrate of the phylum Echinodermata, such as a starfish, sea urchin, or sea cucumber. |
endoskeleton | an internal skeleton, such as the bony or cartilaginous skeleton of vertebrates. |
chordate | an animal of the large phylum Chordata, comprising the vertebrates together with the sea squirts and lancelets. |
notochord | a cartilaginous skeletal rod supporting the body in all embryonic and some adult chordate animals. |
vertebra | each of the series of small bones forming the backbone, having several projections for articulation and muscle attachment, and a hole through which the spinal cord passes. |
ectotherm | an animal that is dependent on external sources of body heat. |
endotherm | an animal that is dependent on or capable of the internal generation of heat; a warm-blooded animal. |
fish | a limbless cold-blooded vertebrate animal with gills and fins and living wholly in water. |
cartilage | firm, whitish, flexible connective tissue found in various forms in the larynx and respiratory tract, in structures such as the external ear, and in the articulating surfaces of joints. |
amphibian | a cold-blooded vertebrate animal of a class that comprises the frogs, toads, newts, and salamanders. They are distinguished by having an aquatic gill-breathing larval stage followed (typically) by a terrestrial lung-breathing adult stage. |
reptile | a vertebrate animal of a class that includes snakes, lizards, crocodiles, turtles, and tortoises. They are distinguished by having a dry scaly skin and typically laying soft-shelled eggs on land. |
bird | a warm-blooded egg-laying vertebrate distinguished by the possession of feathers, wings, and a beak and (typically) by being able to fly. |
mammal | a warm-blooded vertebrate animal of a class that is distinguished by the possession of hair or fur, the secretion of milk by females for the nourishment of the young, and (typically) the birth of live young. |
mammary gland | the milk-producing gland of women or other female mammals. |
monotreme | a primitive mammal that lays large yolky eggs and has a common opening for the urogenital and digestive systems. Monotremes are now restricted to Australia and New Guinea, and comprise the platypus and the echidnas. |
marsupial | a mammal of an order whose members are born incompletely developed and are typically carried and suckled in a pouch on the mother's belly. |
placental mammal | a mammal that develops inside its mother's body until its body systems can function independently |
placenta | a flattened circular organ in the uterus of pregnant eutherian mammals, nourishing and maintaining the fetus through the umbilical cord. |
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