A nocturnal, burrowing, insectivorous, piglike mammal (Orycteropus afer) in the order Tubulidentata. The mammalian order Tubulidentata contains a single family, Orycteropodidae, and a single extant aardvark species, Orycteropus afer (Fig. 1). Aardvarks occur throughout Africa south of the Sahara, wherever suitable habitat exists. In previous classifications, aardvarks and the fossil tubulidentates were assigned to the now-invalid order known as Edentata and were considered closely related to ungulates. However, data from mitochondrial and nuclear genes support a relationship among aardvarks, elephant shrews (family Macroscelididae), paenungulates (hyraxes, sirenians, and proboscideans), and golden moles (family Chrysochloridae). These ecologically divergent adaptive types probably originated in Africa; the molecular evidence implies that they may have arisen there from a common ancestor that existed in the Cretaceous Period, when Africa was isolated from other continents. The members of the order Tubulidentata show the results of an extreme adaptation for burrowing and for feeding on small food items (mainly termites). See also: Adaptation (biology); Africa; African mammals; Edentata; Mammalia; Tubulidentata

A plant, Musa textilis, of the banana family that is valuable for its hard fiber (also known as Manila hemp). Abaca (Musa textilis) is a flowering plant native to the Philippines and Borneo. Taxonomically, it is assigned to the order Zingiberales. Abaca is one of the strongest of the hard fibers. Commercially, it is known as Manila hemp. The fiber is obtained from the leafstalks of M. textilis, which is a member of the banana family. The plant (see illustration) resembles the fruiting banana plant, but it is a bit shorter in stature, bears small inedible fruits, and has leaves that stand more erect than those of the banana. Abaca leaves are also slightly narrower, more pointed, and about 1.5–2 m (4.9–6.6 ft) in length. Relatives of abaca grow wild throughout Southeast Asia, but the plant was domesticated long ago in the southern Philippines. Plants that can yield fiber are grown in a few other parts of the world, chiefly Central America and Ecuador. Most commercial production comes from the Philippines and Ecuador. See also: Banana; Fiber crops; Natural fiber; Zingiberales

Any of the adenosine triphosphate–binding cassette (ABC) transmembrane proteins that use energy to transport various molecules across extracellular and intracellular membranes (cytoplasmic membranes, endoplasmic reticulum, mitochondria, or peroxisomes). ABC lipid transporters consist of two hydrophobic transmembrane domains and two hydrophilic nucleotide-binding folds. These binding folds contain highly conserved sequence motifs (Walker A and B) and are separated by a linker sequence, also known as the signature (C) motif. ABC transporters are organized as full transporters or half transporters depending on the number of transmembrane domains and nucleotide-binding folds. Half transporters must form homodimers or heterodimers to be functional. See also: Adenosine triphosphate (ATP); Lipid; Lipid metabolism; Protein

A major body division of the vertebrate trunk lying posterior to the thorax (chest) and, in mammals, bounded anteriorly by the diaphragm and extending to the pelvis. The abdomen (see illustration) is a major anatomical region of the body. The diaphragm, found only in mammals, separates the abdominal or peritoneal cavity from the pleural and pericardial cavities of the thorax (chest). In all pulmonate vertebrates (possessing lungs or lunglike organs) other than mammals, the lungs lie in the same cavity with the abdominal viscera (internal organs), and this cavity is known as the pleuroperitoneal cavity. See also: Lung; Mammalia; Regional anatomy; Vertebrata

A major plant hormone that plays an important role in plant growth and development. Abscisic acid (ABA), with a chemical formula of C₁₅H₂₀O₄ (Fig. 1), is one of the most significant hormones found in plants. It is responsible for regulating various aspects of plant physiological function, including development, growth, and stress responses. Abscisic acid is ubiquitous in vascular plants; the highest levels are found in young leaves and in developing fruits and seeds. In addition, abscisic acid is produced by mosses, certain algae, several phytopathogenic fungi, and various animals, ranging from sea sponges to mammals. Some abscisic acid signaling mechanisms appear to be common to plants and animals. See also: Plant; Plant development; Plant growth; Plant hormone; Plant metabolism; Plant physiology

The process whereby a plant sheds one of its parts. Leaves, flowers, seeds, and fruits are parts commonly abscised. Almost any plant part, such as very small buds and bracts to branches several inches in diameter, and scales or sheets of bark, may be abscised by some species. However, other species, including many annual plants, may show little abscission, especially of leaves.

The net movement (transport) of water and solutes from outside an organism to its interior. The unidirectional flow of materials into an animal from the environment generally takes place across the alimentary tract, the lungs, or the skin. In each location, a specific cell layer called an epithelium (Fig. 1) regulates the passage of materials. See also: Cell (biology); Epithelium; Respiration

A class of the phylum Radiozoa in the Actinopoda. The kingdom Protozoa contains 18 phyla. One of the parvkingdoms (a hierarchical classification between kingdom and superphylum that is controversial and not officially recognized as such) is the Actinopoda (originally a class) containing two phyla: Heliozoa and Radiozoa. Within the Radiozoa is the class Acantharea. These marine protozoans possess a nonliving, organic capsular wall surrounding a central mass of cytoplasm. The intracapsular cytoplasm is connected to the extracellular cytoplasm by fine cytoplasmic strands passing through pores in the capsular wall. When viewed with the electron microscope, the capsular wall in some species appears to be made of many layers, each composed of a fine fibrillar network. The skeletons are made of celestite (strontium sulfate) instead of silica. The basic structural elements are 20 rods that pass through the capsule to the center in regular arrangements (polar and equatorial; see illustration). An equatorial rod forms a 90° angle with a polar rod, and other groups are arranged with similar exactness. This type of cytoskeleton may be modified by the addition of a latticework, composed of plates, each fused with a skeletal rod. Some genera show a double latticework, concentric with the central capsule. The skeletons do not contribute substantially to the paleontological record in marine sediments since celestite is dissolved by seawater. Dissolution is pronounced below 617 ft (188 m) depth. While the protozoan is still alive, however, the cytoplasm appears to protect the skeleton from dissolution.

An order of ectoparasitic annelid worms consisting of one species, Acanthobdella peledina, which feeds on the dermal tissues of freshwater salmonid fishes in the far regions of Northern Europe. The order Acanthobdellida is taxonomically odd in that it comprises a single family, Acanthobdellidae; a single genus, Acanthobdella; and a single species, Acanthobdella peledina (see illustration). This unusual status reflects the equally unusual combination of characteristics that the worm exhibits, in a manner that reflects the expectation of “transitional forms” that Charles Darwin considered to be a central prediction of his theory on the origin of species. See also: Annelida

A distinct phylum of helminths (parasitic worms), commonly known as the spiny- or thorny-headed worms. The phylum Acanthocephala comprises three classes: Archiacanthocephala, Palaeacanthocephala, and Eoacanthocephala (a suggested fourth class, Polyacanthocephala, has been determined to be part of Eoacanthocephala). The adult members (Fig. 1) of the phylum—termed acanthocephalans, spiny-headed worms, or thorny-headed worms—are parasitic in the alimentary canal of vertebrates. All acanthocephalans are obligatory parasites throughout their entire life cycle; no known member exists as a free-living organism. Approximately 1100 species have been described from all classes of vertebrates, although more species occur in fish (but not elasmobranch fish) than in birds and mammals and only a relatively few species are found in amphibians and reptiles. The geographical distribution of acanthocephalans is worldwide, but genera and species do not have a uniform distribution because some species are confined to limited geographic areas. Host specificity is well established in some species, whereas others exhibit a wide range of host tolerance. The same species never occurs normally, as an adult, in cold-blooded and warm-blooded definitive hosts. The fact that larval development occurs in arthropods gives support to the postulation that the ancestors of acanthocephalans were parasites of primitive arthropods during or before the Cambrian Period and became parasites of vertebrates as this group arose and utilized arthropods for food. Molecular analyses have indicated a close relationship with rotifers (microscopic, mainly free-living aquatic animals), and some researchers have suggested that acanthocephalans and rotifers should be combined into a single taxonomic group (Syndermata). See also: Archiacanthocephala; Arthropoda; Eoacanthocephala; Invertebrate phylogeny; Medical parasitology; Palaeacanthocephala; Parasitology; Rotifera; Vertebrata