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The tailless whip scorpions or whip spiders, an order of the class Arachnida. There are about 80 species in the tropics and subtropics. They are flattened, red to brown, and range from 5 to 45 mm (0.2 to 1.8 in.) in body length. All are nocturnal,…

Any protozoan moving by means of protoplasmic flow. In their entirety, the ameboid protozoa include naked amebas, those enclosed within a shell or test, as well as more highly developed representatives such as the heliozoians, radiolarians, and…

An order of actinopterygian fishes in the subclass Neopterygii. Amiiformes plus several related fossil orders comprise the Halecomorphi, a well-developed group known from the middle Mesozoic. The Amiiformes comprise several families that persisted…

A collective term for the classes Reptilia (reptiles), Aves (birds), and Mammalia (mammals) of the subphylum Vertebrata. The remaining vertebrates, including the several classes of fishes and the amphibians, are grouped together as the Anamnia.…

An order of Lobosia without protective coverings (tests). These protozoa range in size from about 4 micrometers to 0.08–0.12 in. (2–3 mm). Pellicles may be thin, as in Amoeba proteus, or thicker and less flexible, as in Thecamoeba verrucosa (Fig.…

One of four classes (the others are Reptilia, Aves, and Mammalia) composing the superclass Tetrapoda of the subphylum Vertebrata. The living amphibians number over 4700 species and are classified in three orders: Salientia or Anura (frogs and toads,…

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…

Formerly, a group of reptiles that was recognized in rank-based classifications. Amniotes were once divided into subclasses, based on the patterns of temporal fenestrae (openings in the skull roof posterior to the openings for the eyes). Subclass…

An extant order of the crustacean superorder Syncarida confined to freshwater and recorded from both surface and subterranean waters. Members of the order Anaspidacea (anaspidaceans) lack a carapace. The first thoracomere (thoracic somite) is…

The true eels, a large order of actinopterygian fishes, also known as the Apodes. The order Anguilliformes is related to Saccopharyngiformes (sackpharynx fishes), Elopiformes (tarpons), and Notacanthiformes (spiny eels and halosaurs); all members…