Marsupials and Monotremes

Marsupials and Monotremes are two orders of mammal that differ from other mammalian groups in the ways that their young develop. The order Marsupalia, the pouched mammals, is made up of kangaroos and their relatives. Typically, marsupials give birth to their young at a very early stage of development. The young then crawls to the mother’s pouch (which is on the outside of her abdomen), where it attaches itself to a nipple and remains until fully developed. Most marsupials live in Australia, although the opossums—which are classified as marsupials despite not having a pouch—live in the Americas. The order Monotremata is made up of the platypus and its relatives (the echidnas, or spiny anteaters). The monotremes are primitive mammals that lay eggs, which the mother incubates. The monotremes are found only in Australia and New Guinea.

Dolphins, whales, and seals

Dolphins, whales, and seals belong to two orders of mammal adapted to living in water. Dolphins and whales make up the order Cetacea. Typical cetacean features include a streamlined, fishlike shape; forelimbs in the form of flippers; no visible hind limbs; a horizontally flattened tail; and thick blubber under the skin. There are two groups of cetaceans: toothed whales, including sperm whales, white whales, beaked whales, dolphins, and porpoises; and the larger whalebone (baleen) whales, including rorquals, gray whales, and right whales. The blue whale—a rorqual—is the largest living animal: an adult may be up to 100 ft (30 m) long and weigh 145 tons (130 metric tons). Seals and their relatives—sea lions and walruses—make up the order Pinnipedia. Characteristically, they have a streamlined, torpedo-shaped body; forelimbs and hind limbs modified as flippers; thick blubber; and no external ears.

Primates

The mammalian order primates consists of monkeys, apes, and their relatives (including humans). There are two suborders of primates: Prosimii, the primitive primates, which include lemurs, tarsiers, and lorises; and Anthropoidea, the advanced primates, which include monkeys, apes, and humans. The anthropoids are divided into New World monkeys, Old World monkeys, and hominids. New
World monkeys typically have wide-apart nostrils that open to the side; and long tails, which are prehensile (grasping) in some species. This group of monkeys lives in South America, and includes marmosets, tamarins, and howler monkeys. Old World monkeys typically have close-set nostrils that open forward or downward; and non-prehensile tails. This group of monkeys lives in Africa and Asia, and includes langurs, mandrills, macaques, and baboons. Hominids typically have large brains, and no tail. This group includes the apes—chimpanzees, gibbons, gorillas, and orangutans—and humans.

Elephants

The two species of elephant—African and Asian—are the only members of the mammalian order Proboscidea. The bigger African elephant is the largest land animal: a fully grown male may be up to 13 ft (4 m) tall and weigh nearly 8 tons (7 metric tons). A fully grown male Asian elephant may be 11 ft (3.3 m) tall and weigh 6 tons (5.4 metric tons). The trunk—an extension of the nose and upper lip—is the elephant’s other most obvious feature. It is used for manipulating and lifting, feeding, drinking and spraying water, smelling, touching, and producing trumpeting sounds. Other characteristic features include a pair of tusks, used for defense and for crushing vegetation; thick,
pillarlike legs and broad feet to support the massive body; and large ear flaps that act as radiators to keep the elephant cool.

Ungulates

Ungulates is a general term for a large, varied group of mammals that includes horses, cattle, and
their relatives. The ungulates are divided into two orders on the basis of the number of toes. Members
of the order Perissodactyla (odd-toed ungulates) have one or three toes. Perissodactyls include horses,
onagers, and zebras (all of which are one-toed), and rhinoceroses and tapirs (which are three-toed). Members of the order Artiodactyla (even-toed ungulates) have two or four toes. Most artiodactyls
have two toes, which are typically encased in hooves to give the so-called cloven hoof. Two-toed, cloven-hoofed artiodactyls include cows and other cattle, sheep, goats, antelopes, deer, and giraffes. The other main two-toed artiodactyls are camels and llamas. Most two-toed artiodactyls are ruminants; that is, they have a fourchambered stomach and chew the cud. The principal four-toed artiodactyls are pigs, peccaries, and hippopotamuses.

Rabbits and rodents

Although rabbits and rodents belong to different orders of mammals, they have some features in common. These features include chisel-shaped incisor teeth that grow continually, and eating their feces to extract more nutrients from their plant diet. Rabbits and hares belong to the order Lagomorpha. Characteristically, they have four incisors in the upper jaw and two in the lower jaw; powerful hind legs for jumping; forelimbs adapted for burrowing; long ears; and a small tail. Rodents
make up the order Rodentia. This is the largest order of mammals, with more than 1,700 species, including squirrels, beavers, chipmunks, gophers, rats, mice, lemmings, gerbils, porcupines, cavies,
and the capybara. Typical rodent features include two incisors in each jaw; short forelimbs for manipulating food; and cheek pouches for storing food.

Carnivores

The mammalian order carnivora includes cats, dogs,bears, raccoons, pandas, weasels, badgers, skunks, otters, civets, mongooses, and hyenas. The order’s name is derived from the fact that most of its members are carnivores (flesh -eaters). Typical carnivore features therefore reflect a hunting life-style: speed and agility; sharp claws and well-developed canine teeth for holding and killing prey; carnassial teeth (cheek teeth) for cutting flesh; and forward-facing eyes for good distance judgment. However, some members of the order—bears, badgers, and foxes, for example— have a more mixed diet, and a few are entirely herbivorous (plant-eating), notably pandas. Such animals have no carnassial teeth and tend to be slower moving than pure flesh-eaters.

Eggs

An egg is a single cell, produced by the female, with the capacity to develop into a new individual.
Development may take place inside the mother’s body (as in most mammals) or outside, in which case the egg has a protective covering such as a shell. Egg yolk nourishes the growing young. Eggs developing inside the mother generally have little yolk, because the young are nourished from her body. Eggs developing outside may also have little yolk if they are produced by animals whose young go through a larval stage (such as a caterpillar) that feeds itself while developing into the adult form. The shelled eggs of birds and reptiles contain enough yolk to sustain the young until it hatches into a juvenile version of the adult.

Birds

Birds make up the class aves. There are more than 9,000 species, almost all of which can fly (the only flightless birds are penguins, ostriches, rheas, cassowaries, and kiwis). The ability to fly is reflected in the typical bird features: forelimbs modified as wings; a streamlined body; and hollow bones to reduce weight. All birds lay hard-shelled eggs, which the parents incubate. Birds’ beaks and feet vary according to diet and way of life. Beaks range from general-purpose types suitable for a mixed diet (those of thrushes, for example), to types specialized for particular foods (such as the large, curved, sieving beaks of flamingos). Feet range from the webbed “paddles” of ducks, to the talons of birds of prey. Plumage also varies widely, and in many species the male is brightly colored for courtship display whereas the female is drab.

Crocodilians and turtles

Crocodilians and turtles belong to different orders in the class Reptilia. The order Crocodilia includes crocodiles, alligators, caimans, and gharials. Typically, crocodilians are carnivores (flesh-eaters), and have a long snout, sharp teeth for gripping prey, and hard, square scales. All crocodilians are adapted to living on land and in water: they have four strong legs for moving on land; a powerful tail for swimming; and their eyes and nostrils are high on the head so that they stay above water while the rest of the body is submerged. The order Chelonia includes marine turtles, terrapins (freshwater turtles), and tortoises (land turtles). Characteristically, chelonians have a short, broad body encased in a bony shell with an outer horny covering, into which the head and limbs can be withdrawn; and a horny beak instead of teeth.

Lizards and snakes

Lizards and snakes belong to the order Squamata, a division of the class Reptilia. Characteristic reptilian features include scaly skin, lungs, and cold-bloodedness. Most reptiles lay leathery-shelled eggs, although some hatch the eggs inside their bodies and give birth to live young. Lizards belong to the suborder Lacertilia. Typically, they have long tails, and shed their skin in several pieces. Many lizards can regenerate a tail if it is lost; some can change color; and some are limbless. Snakes make up the suborder Ophidia (also called Serpentes). All snakes have long, limbless bodies; can dislocate their lower jaw to swallow large prey; and have eyelids that are joined together to form a single transparent covering over the front of the eye. Most snakes shed their skin in a single piece. Constrictor snakes kill their prey by squeezing; venomous snakes poison their prey.

Amphibians

The class amphibia includes frogs and toads (which make up the order Anura), and newts and salamanders (which make up the order Urodela). Amphibians typically have moist, scaleless, hairless skin; lungs; and are cold-blooded. They also undergo complete metamorphosis, from eggs laid in water through various water-living larval stages (such as tadpoles) to land-living adults. Typical features of adult frogs and toads include a squat body with no tail; long, powerful hind legs; and large, often bulging, eyes. Adult newts and salamanders typically have a long body with a well developed tail; and relatively short, equal-sized legs. However, newts and salamanders show considerable variation; for example, in some species the adults have minute legs, external gills rather than lungs, and spend their entire lives in water.

Bony fish

Bony fish, such as carp, trout, salmon, perch, and cod, are by far the best known and largest group of fish, with more than 20,000 species (over 95 percent of all known fish). As their name suggests, bony fish have skeletons made of bone, in contrast to the cartilaginous skeletons of sharks, jawless fish, and their relatives . Other typical features of bony fish include a swim bladder, which functions as a variable-buoyancy organ, enabling a fish to remain effortlessly at whatever depth it is swimming; relatively thin, bonelike scales; a flap (called an operculum) covering the gills; and paired pelvic and pectoral fins. Scientifically, bony fish belong to the class Osteichthyes, which is a division of the superclass Gnathostomata (meaning “jawed mouths”).

Sharks and jawless fish

Sharks,dogfish, skates, and rays belong to a class of fishes called Chondrichthyes, which is a division of the superclass Gnathostomata (meaning “jawed mouths”). Also sometimes known as elasmobranchs, sharks and their relatives have a skeleton made of cartilage (hence their common name, cartilaginous fish), a characteristic that distinguishes them from bony fish. Other important features of cartilaginous fish are extremely tough, toothlike scales, and lack of a swim bladder. Jawless fish— lampreys and hagfish—are primitive, eellike fish that make up the order Cyclostomata (meaning “round mouths”), a division of the superclass Agnatha (meaning “without jaws”). In addition to their characteristic round, suckerlike mouths and lack of jaws, cyclostomes also have smooth, slimy skin without scales, and unpaired fins.

Mollusks

The phylum mollusca is a large group of animals that includes octopuses, snails, and scallops. Octopuses and their relatives— including squid and cuttlefish—form the class Cephalopoda. Cephalopods typically have a head with a radula (a filelike feeding organ) and beak; a well-developed nervous system; sucker-bearing tentacles; a muscular mantle (part of the body wall) that can expel water through the siphon, enabling movement by jet propulsion; and a small shell or no shell. Snails and their relatives—including slugs, limpets, and abalones—make up the class Gastropoda. Gastropods typically have a coiled external shell, although some, such as slugs, have a small internal shell or no shell; a flat foot; and a head with tentacles and a radula. Scallops and their relatives—including clams, mussels, and oysters—make up the class Bivalvia (also called Pelecypoda). Features of bivalves include a shell with two halves (valves); large gills that are used for
breathing and filter feeding; and no radula.

Starfish and sea urchins

Starfish,sea urchins,and their  eather stars, brittle stars, basket stars, sea daisies, sea lilies, and sea cucumbers) make up the phylum Echinodermata. A unique feature of echinoderms is the water vascular system, which consists of a series of water-filled canals from which protrude thousands of tiny tube feet. The tube feet may be used for movement, feeding, or respiration. Other features include pentaradiate symmetry (that is, the body can be divided into five parts radiating from the center); no head; a diffuse, decentralized nervous system that lacks a brain; and no excretory organs. Typically, echinoderms also have an endoskeleton (internal skeleton) consisting of hard calcite ossicles embedded in the body wall and often bearing protruding spines or tubercles. The ossicles may fit together to form a test (as in sea urchins) or remain separate (as in sea cucumbers).

Crustaceans

The subphylum crustacea is one of the largest groups in the phylum Arthropoda. The subphylum is divided into several classes, the most important of which are Malacostraca and Cirripedia. The class
Malacostraca includes crayfish, crabs, lobsters, and shrimps. Typical features of malacostracans include a body divided into two sections (a combined head and thorax called a cephalothorax, and an abdomen); an exoskeleton (external skeleton) with a large plate (carapace) covering the cephalothorax; stalked, compound eyes; and two pairs of antennae. The class Cirripedia includes barnacles, which, unlike other crustaceans, spend their adult lives attached to a surface, such as a rock. Other characteristics of cirripedes include an exoskeleton of overlapping calcareous plates; a body consisting almost entirely of thorax (the abdomen and head are minute); and six pairs of thoracic appendages (cirri) used for filter feeding.

Arachnids

The class arachnida includes spiders (order Araneae) and scorpions (order Scorpiones). The class is part of the phylum Arthropoda, which also includes insects and crustaceans. Spiders and scorpions are characterized by having four pairs of walking legs; a pair of pincerlike mouthparts called chelicerae; another pair of frontal appendages called pedipalps, which are sensory in spiders but used for grasping in scorpions; and a body divided into two sections (a combined head and thorax called a cephalothorax or prosoma, and an abdomen or opisthosoma). Unlike other arthropods, spiders and
scorpions lack antennae. Spiders and scorpions are carnivorous. Spiders poison prey by biting with the fanged chelicerae, scorpions by stinging with the end of the metasoma (tail).

Insects

The word insect refers to small invertebrate creatures, especially those with bodies divided into sections. Insects, including beetles, ants, bees, butterflies, and moths, belong to various orders in the class Insecta, which is a division of the phylum Arthropoda. Features common to all insects are an
exoskeleton (external skeleton); three pairs of jointed legs; three body sections (head, thorax, and abdomen); and one pair of sensory antennae. Beetles (order Coleoptera) are the biggest group of insect, with about 300,000 species (about 30 percent of all known insects). They have a pair of hard elytra (wing cases), which are modified front wings. The principal function of the elytra is to protect the hind wings, which are used for flying. Ants, together with bees and wasps, form the order Hymenoptera, which contains about 200,000 species. This group is characterized by a marked narrowing between the thorax and abdomen. Butterflies and moths form the order Lepidoptera, which has about 150,000 species. They have wings covered with tiny scales, hence the name of their order (Lepidoptera means “scale wings”). The separation of lepidopterans into butterflies and moths is largely artificial, since there are no features that categorically distinguish one group from the other. In general, however, most butterflies fly by day, whereas most moths are night-flyers. Some insects, including butterflies and moths, undergo complete metamorphosis (transformation) during their life-cycle. A butterfly metamorphoses from an egg to a larva (caterpillar), then to a pupa (chrysalis), and finally to an imago (adult).

Sponges, jellyfish, and sea anemones

Sponges are mainly marine animals that make up the phylum Porifera. They are among the simplest of all animals, having no tissues or organs. Their bodies consist of two layers of cells separated by a
jellylike layer (mesohyal) that is strengthened by mineral spicules or protein fibers. The body is perforated by a system of pores and water channels called the aquiferous system. Special cells (choanocytes) with whiplike structures (flagella) draw water through the aquiferous system, thereby bringing tiny food particles to the sponge’s cells. Jellyfish (class Scyphozoa), sea anemones (class Anthozoa), and corals (also class Anthozoa) belong to the phylum Cnidaria, also known as Coelenterata. More complex than sponges, coelenterates have simple tissues, such as nervous tissue; a radially symmetrical body; and a mouth surrounded by tentacles with unique stinging cells (cnidocytes).