A collective term, also referred to as Anamniota, for the vertebrate animals that lack an amnion in development, including the Agnatha, Chondrichthyes, Osteichthyes, and Amphibia. The amnion is a protective embryonic envelope that encloses the embryo and its surrounding liquid, the amniotic fluid, during fetal life. An amnion is present in mammals, birds, and reptiles, but is absent in fishes and amphibians. In early classifications, the vertebrates were commonly separated on this basis, and the expressions Amniota and Anamnia (Anamniota) are still useful in grouping higher and lower vertebrates. It should be recognized that these terms represent grades of development, however, and do not carry the connotation of established classificatory ranks. Anamnia, then, is a group name that includes the Holocene (Recent) members of the Agnatha (jawless fishes), Chondrichthyes, Osteichthyes, and Amphibia; by presumption, the class Placodermi, which is known only from fossils, should be included in the Anamnia as well. See also: Amnion; Amniota; Amphibia; Chondrichthyes; Jawless vertebrates; Osteichthyes; Placodermi; Vertebrata

Any living organism taxonomically assigned to the classification kingdom known as Animalia. Members of the kingdom Animalia—that is, animals (see illustration)—possess certain characteristics that distinguish them from organisms belonging to the other kingdoms—Plantae, Fungi, Protista, and Monera (in certain systematic schemes, the Monera kingdom is replaced by the Archaea and Bacteria kingdoms). However, there is no single criterion that can be used to distinguish all animals from all other organisms. Unlike plants, for example, animals usually lack chlorophyll and the ability to manufacture foods from raw materials available in the soil, water, and atmosphere. Animal cells are usually delimited by a flexible plasma or cell membrane rather than a cell wall composed either of cellulose or chitin, as are the cells of most plants. Animals generally are limited in their growth, and most have the ability to move in their environment at some stage in their life history. See also: Animal kingdom; Cell membrane; Chlorophyll; Plant; Plant kingdom

A discipline within the field of animal behavior that focuses on the reception and use of signals. Animal communication could well include all of animal behavior because a liberal definition of the term signal could encompass all stimuli perceived by an animal. However, most research in animal communication deals only with those cases in which one member of a species generates a signal, defined as a structured stimulus, that subsequently influences the behavior of another member of the same species in a predictable way (intraspecific communication) [Fig. 1]. In this context, communication occurs in virtually all animal species, if only as a means by which a member of one sex finds its partner. In general, the field of animal communication includes an analysis of the physical characteristics of those signals believed to be responsible in any given case of information transfer. See also: Ethology; Instinctive behavior; Neuroethology; Physiological ecology (animal); Primate communication

An animal's movement through the atmosphere sustained by aerodynamic reaction or other forces. Animal flight includes gliding and flapping flight. Gliding flight does not involve the generation of thrust, whereas flapping flight does generate thrust via the beating of wings, which are thin structures that produce lift. Four animal groups evolved flapping flight, namely insects, pterosaurs, birds (Fig. 1), and bats. Flapping flight in vertebrates was probably preceded by gliding; in insects it may have originated by leaping and gliding, by surface skimming on water, or (if small enough) by passive floating in the air. Flying insects show greater variation than flying vertebrates, and their flight spans a wider range of Reynolds numbers, which is the ratio of inertial forces to viscous forces in the flow. Flight of tiny insects is in the lower range of Reynolds numbers, where viscous forces are dominant, whereas large insects and vertebrates operate in the higher range, where inertial forces are important. See also: Animal; Atmosphere; Aves; Chiroptera; Insect flight muscle; Insecta; Pterosauria; Reynolds number; Wing

The classification rank comprising the animals. Animals are taxonomically assigned to the classification kingdom known as Animalia. (The other kingdoms for classification of organisms include Plantae, Fungi, Protista, and Monera; in certain systematic schemes, the Monera kingdom is replaced by the Archaea and Bacteria.) Animals are eukaryotic multicellular organisms that take food into their bodies and that develop from blastula embryos. The animal kingdom includes more than 1.2 million described species. Animal species are organized into phyla (see illustration) that are defined according to comparative patterns of development, body structures, behavior, biochemical pathways, modes of nutrition, and ancestry. See also: Biological classification; Eukaryota; Metazoa; Systematics; Taxonomic categories; Taxonomy; Zoology

Any terrestrial or aquatic animal that is capable of excavating holes in the ground for protection from adverse environmental conditions, as well as for storing food. Burrowing animals are uniquely adapted species, having the capacity to dig burrows for protective purposes, as well as for nesting, hibernation, warmth, and food storage. Burrows vary from temporary structures of simple design (for example, the nesting burrows of some birds) to more permanent underground networks that may be inhabited for several generations (for example, rabbit warrens, badger sets, fox earths, and prairie dog burrows; Fig. 1). They vary in structure from blind burrows with a single opening to extensive systems with several openings. Some animals (for example, various species of moles) live permanently underground, and their burrows have no obvious large openings to the surface. Burrows may be shared by a number of species, and abandoned burrows may be used by other species. Animals with limbs usually excavate their burrows by using their legs, but many burrowing animals are limbless and the mechanism of progression is not always obvious. See also: Adaptation (biology); Ecology; Hibernation and estivation

A subphylum of the phylum Chordata comprising the lancelets, including Branchiostoma (amphioxus). Cephalochordates (also known as the Leptocardii) are small fishlike animals, not exceeding 80 mm (3.2 in.) in length. They burrow in sand on the ocean bed or in estuaries in tropical and temperate regions throughout the world. Three genera are recognized in Cephalochordata: Branchiostoma with 23 species, Epigonichthys with 5 species, and Asymmetron with 2 species. See also: Chordata

The phylum of animals characterized by possession of a notochord, which includes the Cephalochordata (lancelets), Urochordata (tunicates), and Craniata (vertebrates and hagfishes). The phylum Chordata also may include the extinct Yunnanozoons and Vetulicolians. Members of the first two groups are small and strictly marine. The vertebrates include the fishes, amphibians, reptiles, birds, and mammals; they are usually free-living and are found in many environments from marine waters to freshwaters and in almost all terrestrial biomes. Some workers include the acorn worms, pterobranchs, and graptolites (Hemichordata) as a subphylum within Chordata, but here it is considered as a separate phylum because recent morphological and molecular data indicate these organisms are closely related to echinoderms and do not possess a notochord. However, the phylogenetic position of these animals is still controversial. See also: Cephalochordata; Hemichordata; Phylogeny; Tunicata (Urochordata); Vertebrata

A major division of the animal kingdom comprising the phyla Hemichordata, Echinodermata and Chordata. Hemichordata consists of acorn worms (Enteropneusta) and a group of small-bodied, tentaculate forms (Pterobranchia). Echinodermata contains the starfish, sea urchins, crinoids (sea lilies) and their allies. The phylum Chordata includes the Urochordata (sea squirts and larvaceans), Cephalochordata (Branchiostoma, the sand lancelet, formerly Amphioxus) and vertebrates (see illustration). At one time, up to 10 phyla were included in Deuterostomia on the basis of shared morphological and developmental features; however, comparison of deoxyribonucleic acid (DNA) sequences indicates that most of those phyla are more closely allied to other groups. See also: Animal kingdom

A phylum comprising approximately 100 described species of microscopic parasites that live in the renal sacs (kidneys) of many species of octopuses, some cuttlefishes and a few squids. Both Dicyemida and Rhombozoa are used to describe this phylum.