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The common name applied to certain fishes in the order Clupeiformes. Herring is not a taxonomic unit of fishes; therefore, it cannot be precisely defined. The typical image of herrings is a composite of the species in the family Clupeidae (Fig. 1);…

One of the evolutionary lineages of gastropods (class Gastropoda), comprising a very heterogeneous group of snails with forms that resemble typical marine snails with external spiraled shells such as sundial shells (family Architectonicidae), sea…

A small, extinct late Paleozoic order of fossil corals known from Europe, North Africa, Asia, North America, and Australia but limited to the Middle Devonian and to the Late Mississippian. They are found in calcareous shales and in limestones. The…

The largest subclass in the class Bivalvia, phylum Mollusca. The subclass Heterodonta contains some of the most ecologically important bivalves (Fig. 1). For example, marine clams, cockles, and shipworms are heterodonts. Formerly, the class Bivalvia…

An order of galeomorph sharks that are commonly known as the bullhead sharks. Sharks belonging to the order Heterodontiformes are distinguished by the following combination of characters: a typically sharklike (subcylindrical) trunk; a head that is…

A large order of the Spirotrichia containing many well-known, sizable species. The buccal ciliature, both membranes and the adoral zone of membranelles, is well developed, although in a number of families the somatic, or body, ciliature is really…

A subclass of cnidarian class Anthozoa that includes stony and black corals and sea anemones; also known as Zoantharia. With few exceptions, these animals are benthic as adults, living attached to firm substrata or burrowed into soft sediments; they…

An order of squalomorph sharks commonly known as the six- or seven-gill sharks. The order Hexanchiformes comprises two extant families (Chlamydoselachidae, the frill sharks; and Hexanchidae, the cow sharks), four genera, and six species. These…

An order of freshwater actinopterygian (ray-finned) fishes comprising the mooneyes and goldeyes. The order Hiodontiformes consists of one family, Hiodontidae; one genus, Hiodon; and two extant species, H. alosoides (goldeye) and H. tergisus…

The name for two species of even-toed (artiodactylid) ungulates that form the family Hippopotamidae. Both mammalian species of hippopotamus (often termed hippo) occur in Africa. The great African hippopotamus (Hippopotamus amphibius; see…

The leeches, classified as a subclass of worms in the phylum Annelida (class Clitellata). Taxonomically, Hirudinida (also Hirudinea) is considered to be a subclass in the annelid class Clitellata (which also contains the subclass Oligochaeta and the…

An order of irregular echinoids (sea urchins) of the superorder Atelostomata (class Echinoidea), with a strongly bilaterally symmetrical test. Members of the Holasteroida have a small, oval mouth (lacking buccal notches) that lies close to the…

An extinct order of primitive irregular sea urchins of the class Echinoidea, which retain a functioning jaw apparatus throughout life. Although the periproct opens on the oral surface in the posterior interambulacrum of members of the Holectypoida,…

An unranked group or infraclass of the fish subclass Neopterygii (class Actinopterygii) consisting of several fossil orders and the extant orders, Lepisosteiformes and Amiiformes. Members of the Holostei (holosteans) are descended from the older…

A class of Echinozoa characterized by a cylindrical body and smooth leathery skin, which are known as sea cucumbers. In Holothuroidea, there are no arms, but a ring of five or more tentacles may surround the mouth, which is usually at one end of the…

Enigmatic extinct echinoderms whose members are characterized by a flattened body form and the complete lack of pentameral (five-sided radial) symmetry. Previously considered the fifth subphylum of echinoderms, homalozoans (commonly called carpoids)…

A subclass of Crustacea, including a single extant order, Stomatopoda, commonly known as mantis shrimps. The Hoplocarida was formerly included as a taxon within the Eumalacostraca, and disagreement regarding its independent origin still persists.…

A vernacular name applied to plantlike polypoid stages of certain hydrozoan animals (phylum Cnidaria). Hydroids are predominantly bottom-dwelling, sedentary, colonial, and polymorphic. Two major groups are recognized: Anthoathecata (Fig. 1) and…

A class of the aquatic animal phylum Cnidaria, comprising hydroids and hydropolyps (for example, Hydra), veiled and mostly small jellyfishes or hydromedusae (for example, freshwater jellyfish, Craspedacusta), and siphonophores (for example,…

An African carnivore belonging to the family Hyaenidae. The family Hyaenidae (order Carnivora) comprises three species of hyenas (also spelled hyaenas): one species in the genus Crocuta (see illustration) and two species in the genus Hyaena. In…

An order of ciliated protozoans. The order Hymenostomatida belongs to the protozoan class Oligohymenophorea (phylum Ciliophora) and contains many species that often are of small size and fairly uniform ciliation. Primarily, the members of the order…

An order of the Protozoa in the class Zoomas­ti­gophorea comprising the most complex flagellates, both structurally and in modes of division. All inhabit the alimentary canal of termites, cockroaches, and woodroaches. These organisms are…

An order of Spirotrichia. These protozoa are commonly considered to represent the pinnacle of specialized development in the evolution of all ciliates. Somatic ciliature of the ventral surface has been replaced by cirri; on the dorsal surface it is…

An order of mammals closely related to elephants. Hyracoids were a diverse and successful group of mammals in Africa during the Eocene and Oligocene epochs (early part of the Age of Mammals, 55 to 34 million years ago), and they are still…

Formerly, one of the two classes of the phylum Brachiopoda. The phylum comprises solitary, exclusively marine, coelomate [having a true coelom or mesodermally lined body cavity], lophophorate [having a lophophore, a food-gathering ciliated organ],…

Diseases that affect insects (class Insecta). Insect pathology draws upon and contributes to the general field of microbiology and provides understanding of certain of the biological relationships existing between insects and microorganisms not…

The study of the functional properties of insect tissues and organs. Insects are one of the most diverse groups of animals on Earth. They have evolved adaptations that allow them to live in a variety of ecosystems. The basic body plan comprises a…

A class of the phylum Arthropoda, sometimes called the Hexapoda. In fact, Hexapoda is a superclass consisting of both Insecta and the related class Entognatha (Parainsecta) (containing the springtails, telsontails, and proturans). Class Insecta is…

An obsolete order of placental insectivorous mammals, now mostly reassigned to the order Eulipotyphla, including the shrews, moles, hedgehogs, and solenodons. The order Insectivora was formerly thought by taxonomists to be a basal placental order,…

Conservationists are alarmed by significant reductions in insect populations worldwide. Based on recent biodiversity surveys and entomological studies, scientists estimate that more than 40% of the world's insect species (class Insecta, phylum…

An order of nematodes comprising the single superfamily Isolaimoidea. The order consists of one family and one genus. They are rather large for free-living soil nematodes (0.1–0.2 in. or 3–6 mm) and are found in seldom-cultivated sandy soils. Some…

An order of invertebrate malacostracan crustaceans, including aquatic and terrestrial forms. The members of the order Isopoda (subclass Eumalacostraca, class Malacostraca, subphylum Crustacea, phylum Arthropoda) are commonly termed isopods. The…

A suborder of the Acari (Acarina), class Arachnida, comprising the ticks. Ticks differ from mites, their nearest relatives, in their larger size and in having a pair of breathing pores, or spiracles, behind the third or fourth pair of legs. They…

The common name for members of the agnathans. The superclass Agnatha (subphylum Vertebrata, phylum Chordata) comprises the jawless vertebrates (agnathans). Members include the cyclostomes (modern lampreys and hagfishes), as well as extinct armored…

The common name for 33 species of rodents in the family Dipodidae. Jerboas (see table) inhabit the hot, arid desert and semiarid regions of Turkey, North Africa, and Asia. Desert Mice and rats Rodentia Genera and common names of…

A predatory, cosmopolitan cetacean mammal, Orcinus orca, found typically in cold ocean waters. Killer whales (Orcinus orca) [Fig. 1] are top (apex) predators in the marine ecosystem, with no natural enemies other than humans. These carnivorous…

A single species, Phascolarctos cinereus, which is the sole extant member of the family Phascolarctidae in the mammalian order Marsupialia (pouch-bearing animals). The koala (see illustration) is a small, clean, affectionate animal that weighs from…

The dominant group (class) of vertebrates. Members of the class Mammalia (subphylum Vertebrata, phylum Chordata), that is, mammals, have ruled the planet Earth since the extinction of the dinosaurs approximately 65 million years ago. There are…

A member of the genus Marmota in the squirrel family Sciuridae. Fourteen species occur mainly in mountainous areas in the Northern Hemisphere (see table). Rodentia Squirrel Members of the genus Marmota and their geographical…

An order of animals, long considered the only order of the mammalian infraclass Metatheria. Morphology The marsupials are characterized by the presence of a pouch (marsupium) in the female, a skin pocket whose teat-bearing abdominal wall is…

Any of several species of carnivorous, weasel-like mammals in the family Mustelidae. Martens are members of the genus Martes in the mammalian family Mustelidae (order Carnivora). This family also includes weasels, minks, wolverines, otters, ferrets…

A class of Crustacea whose application is gaining moderately widespread use, despite the fact that its validity is not universally accepted by researchers. The constituent subclasses of the Maxillopoda also remain unsettled: before 1956, all major…

A small order of insects called the scorpion flies. Characteristic of the adult insect is the peculiar prolongation of the head into a beak, which bears chewing mouthparts. Members of the order Mecoptera (mecopterans) are small to medium in size.…

A small order considered among the most primitive holometabolous insects. Along with Neuroptera and Raphidioptera, Megaloptera forms part of the superorder Neuropterida, which is recognized generally as a sister group to Coleoptera. Despite a low…

A class of phylum Arthropoda, subphylum Chelicerata. Merostomes are aquatic chelicerates, characterized by abdominal appendages bearing respiratory organs. Most merostomes are extinct; only the horseshoe crabs, comprising four species and three…

An obsolete term formerly used to describe a phylum of microscopic marine parasites. The name Mesozoa—the “in-between animals”—was for many years applied to the parasitic marine invertebrates called Dicyemida (Rhombozoa) and Orthonectida. These two…

A pronounced change in both the internal and external morphology of an animal that takes place in a short amount of time, triggered by some combination of external and internal cues. The extent of morphological change varies considerably among…

An infraclass of therian mammals including a single order, the Marsupialia. The Metatheria are distinguished from the Eutheria (the placental mammals) by numerous characters. The full metatherian dentition is I 5/4 C 1/1 Pm 3/3 M 4/4, for a total of…

The monophyletic group of eukaryotic organisms comprising all multicellular animals, but not those in which their cells form colonies, such as choanoflagellates. In colonies, all the cells need to feed because they are not in contact with each…

A group of ciliated microscopic animals living in freshwater. Micrognathozoa (micro, gnathos, and zoa are Greek for small, jaws, and animal, respectively) was proposed to constitute a new class to accommodate a new species, Limnognathia maerski (see…