Species

Species, in biology, a category of classification, the original and still the basic unit in the demarcation of plant and animal types. The species marks the boundary between populations of organisms rather than between individuals. Because related species are not absolutely permanent (see evolution), a precise definition of the term is difficult. On the basis of genetics, scientists now include in a species all individuals that are potentially or actually capable of interbreeding and that share the same gene pool. The latter term refers to that collection of characteristics whose combination is unique in the species, although each individual of the group may not display every single one of the characteristics (see genetics). In the few cases where members of different species can interbreed, the offspring are usually sterile (e.g., the mule). Groups distinguished by lesser differences than those marking a species are called variously subspecies, varieties, races, or tribes.

See: Species and Speciation

Mammoth

Mammoth, name for several large prehistoric elephants of the extinct genus Mammuthus, which ranged over Eurasia and North America in the Pleistocene epoch. The shoulder height of the Siberian, or woolly, mammoth, which roamed throughout the Northern Hemisphere, was about 9 ft (2.7 m), and that of the imperial mammoth of the North American Great Plains was up to 13 1-2 ft (4.1 m). Mammoths were covered by a long, shaggy, black outer coat and a dense, woolly undercoat. They had complex, many-ridged molar teeth; long, slender upward-curved tusks; and a long trunk. Ivory hunters have collected their tusks for centuries in Siberia, where some 50,000 have been discovered; it is from these and from the drawings left by the Cro-Magnon people in the caves of S France that the mammoth's appearance is known. Paleolithic (Old Stone Age) people hunted mammoths, as is evidenced by remains of the animals found together with tools, and may have contributed to their extinction. Mammoths are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Proboscidea, family Elephantidae.

Photos of Mammals

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Aardwolf
The aardwolf, a member of the hyena family, is found throughout southern and eastern Africa. Aardwolves travel at night hunting for food, which consists of carrion (dead and decaying animal flesh) and insects, usually termites. They have weak jaws and small teeth, leaving them without defenses against enemies such as dogs.

Abyssinian
Abyssinians are particularly affectionate and have fine, soft fur resembling that of a wild rabbit. The origin of the cat is unknown, although its appearance suggests a relationship to the cats of ancient Egypt. It is one of the oldest breeds of domesticated cat.




Afghan Hound
The Afghan hound, similar to the greyhound, originated in Afghanistan and was known in ancient Egypt. A capable hunting dog, the Afghan hound is a swift runner and a powerful jumper.








African Elephants
African elephants live in grassy regions south of the Sahara Desert. Bull (male) elephants stand about 3.4 m (11 ft) tall and weigh about 5.4 metric tons, while cow (female) elephants stand 2.8 m (9 ft) tall and weigh about 3.6 metric tons. They have no sweat glands, so they like to cool off by rolling in ponds and streams. The mud that dries on their skin protects it from the sun.



Agouti
A large rodent common to the forests of Central America and central-northern South America, the swift, ground-dwelling agouti feeds on leaves, roots, and fallen fruit. The agouti is hunted by farmers because it destroys crops in agricultural areas.






Akhal-Teké
A prized desert horse, the Akhal-Teké has been bred and raised for at least 3000 years, and may have been an ancestor of the Arabian. Remarkable endurance and resistance to heat give the powerful yet graceful Akhal-Teké an edge in competitive long-distance riding and show jumping.



Alaskan Malamute
This Alaskan Malamute has dense, layered fur as a shield from severe weather. The nomadic Mahlemuts, an Inuit people of northwestern Alaska, first bred the Alaskan Malamute as a sled dog to haul their belongings. Ranging from light gray to black in color, Alaskan Malamutes are also popular as pets and show dogs.







Alpine Ibex
Famous for its surefootedness in steep, rocky terrain, the Alpine ibex is one of four species of mountain goat in the genus Capra. The Alpine ibex was on the brink of extinction from overhunting and habitat destruction in the early 20th century, but is now protected and rebuilding herd populations in Italy’s Gran Paradiso National Park and other European wildlife preserves.





American Black Bear
The American black bear lives in forests throughout North America, from Alaska and Canada to mountains in northern Mexico. Unlike most other bear species, the American black bear has adapted to human encroachment on its habitat, and it is able to survive in populated suburban areas. Despite the bear’s common name, the color of the American black bear ranges from black to brown, cinnamon, beige, and even pure white.






Antarctic Fur Seal
The Antarctic fur seal is one of 14 species of eared seals. Fur seals are polygamous and assemble in huge breeding colonies beginning in October. Breeding females feed exclusively on small, shrimplike crustaceans called krill, whereas males feed on krill, penguins, and fish. Although most eared seal populations have been decimated by overhunting, the Antarctic fur seal has rebounded from near extinction in the late 1800s to a current population of between 700,000 and 1 million animals.

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Coral

Coral, common name for members of a large class of marine invertebrates characterized by a protective calcium carbonate or horny skeleton. This protective skeleton is also called coral. Corals are divided into two subclasses, based on differences in their radial symmetry (symmetry around a central axis). One subclass consists of colonial, eight-tentacled animals, each with an internal skeleton. Among them are whip corals, gorgonians, and the red coral used in making jewelry. Members of the other subclass commonly have six tentacles, or multiples of six, but other patterns are also observed. They include the stony, or true, corals. Another class of the same phylum also contains forms known as coral that are not considered here.

STRUCTURE
True corals secrete calcium carbonate from the bottom half of the stalk of the individual animal, or polyp, forming skeletal cups to which the polyps are anchored and into which they withdraw for protection. In the flattened oral disc at the top of the stalk is an opening, edged with feathery tentacles and cilia, that is both mouth and anus. At night the tentacles extend from the cup, seize animal plankton that wash against them, and carry the food to the mouth. Stinging cells, or nematocysts, on the tentacles can also paralyze prey.

COLONIES
Some scleractinians exist as solitary polyps, but the majority are colonial. Colonial polyps average from 1 to 3 mm (0.04 to 0.12 in) in diameter. They are connected laterally by tubes that are an extension of the polyps' gastrovascular cavities, and the colony grows by asexual budding from the base or the oral disc of the polyps. Living polyps build on the deposits of their predecessors; the wide range of branched or massive forms that result depends on the species involved.

Colonial corals can grow in deep water, but reef-building corals are found only in warm, shallow seas. They live no deeper than light can penetrate because the symbiotic algae called zooxanthellae that live in their tissues require light for photosynthesis, and the corals cannot exist without the algae. Carbon is passed by the algae to the coral, increasing its energy, and the food caught by the coral may supply nitrogen and phosphorus for both organisms. The dependence of the corals on the algae probably varies according to species and locality. Scleractinian corals are the main contributors to a reef, but other organisms also add to it, such as hydrozoan corals, calcareous algae, mollusks, and sponges. See Coral Reef.

Scientific classification: Corals belong to the class Anthozoa in the phylum Cnidaria. Corals with eight tentacles make up the subclass Ostocorallia or Alcyonaria. Corals that commonly have six tentacles or multiples of six belong to the subclass Zoantharia (or Hexacorallia). Stony, or true, corals belong to the order Scleractinia (or Madreporaria).

Silkworm

Silkworm, common name for the silk-producing larvae of any of several species of moths. Silkworms possess a pair of specially modified salivary glands called silk glands, or sericteries, which are used in the production of cocoons. The silk glands secrete a clear, viscous fluid that is forced through openings, called spinnerets, on the mouthparts of the larva; the fluid hardens as it comes into contact with air. The diameter of the spinneret determines the thickness of the silk thread produced.

The best-known silkworm is the larvae of the common, domesticated silkworm moth. This moth, native to China, was introduced into Europe and western Asia in the 6th century ad and into North America in the 18th century. The moth has been cultivated for many centuries and is no longer known in the wild state. Breeders have produced many varieties of the moth, the most important of which produce three broods of young annually.

A typical adult silkworm moth is yellow or yellowish-white, with a thick, hairy body, and has a wingspread of about 3.8 cm (about 1.5 in). The adult has rudimentary mouthparts and does not eat during the short period of its mature existence; the female dies almost immediately after depositing the eggs, and the male lives only a short time thereafter. The female deposits 300 to 400 bluish eggs at a time; the eggs are fastened to a flat surface by a gummy substance secreted by the female. The larvae, which hatch in about ten days, are about 0.6 cm (about 0.25 in) long. The larvae feed on leaves of white mulberry, Osage orange, or lettuce. Silkworm caterpillars (see Caterpillar) that are fed mulberry leaves produce the finest quality silk. Mature larvae are about 7.5 cm (about 3 in) long and yellowish-gray or dark gray in color.

About six weeks after hatching, the common silkworm stops eating and spins its cocoon. The length of the individual fiber composing the cocoon varies from 300 to 900 m (1000 to 3000 ft). The silkworm pupates for about two weeks; if allowed to complete its pupation period, it emerges as an adult moth. Tearing during emergence damages the silken cocoon beyond commercial use. Therefore, in the commercial production of silk, only enough adult moths are allowed to emerge to ensure continuation of the species. Most of the silkworms are killed by heat, either by immersion in boiling water or by drying in ovens.

Other moths known as silkworm moths include the giant silkworm moths. The larvae of these large moths also spin silken cocoons, but they are less widely used for commercial silk production.

Scientific classification: Silkworms belong to the order Lepidoptera. The domesticated silkworm moth makes up the family Bombycidae and is classified as Bombyx mori. The giant silkworm moths make up the family Saturniidae.