Animal kingdom

Human perception of the animal kingdom tends to focus on relatively large vertebrates. However, these large vertebrates are true minorities, accounting for just a tiny fraction of the animal world. Over 97 percent of animal species are invertebrates, the earliest animals to emerge. Insects and arthropods make up the vast majority of animal species and a huge percentage of the individual animals on Earth. Most other animal phyla are also far more diverse and numerous than vertebrates. All vertebrates together constitute only part of a single phylum, Chordata. In simple terms, the small and boneless creatures called invertebrates dominate the animal kingdom. They live bountifully in diverse habitats: in pond muck, on ocean bottoms, in treetops, beneath leaf litters, and in many other environments.

Animals are easy to identify but difficult to define due to the diversity and complexity of all creatures in this kingdom. The best approach relies on a set of common characteristics that distinguish animals from individuals of other kingdoms, a field called systematics. First, animals are multicellular (made up of many cells). Second, animals are heterotrophic, obtaining nutrients and energy by consuming other organisms. Third, animals are usually capable of sexual reproduction, although other reproductive styles may exist. Fourth, animal cells contain no cell wall. Fifth, animals are mobile during at least some stage of their lives. Finally, animals are usually capable of rapidly responding to external stimuli through their nerve cells, muscle, or contractile tissue. These six characteristics, taken together, distinguish animals from other living creatures.

Based on evolutionary theories, animal phyla show trends toward increasing cellular organization and complexity. In the most ancient phylum of animals, sponges, individual cells may have specialized functions but act independently, and hence, are not organized into tissues or organs. Cnidarians (jellyfish and their relatives), the phylum most closely related to sponges, have well-defined tissues that coordinate movement and sensory information. Flatworms, the next phylum to emerge, have organs and organ systems, such as a reproductive system. Organ systems are also found in all the remaining, more recently emerged animal phyla. The trend toward increasing complexity goes beyond the level of cellular organization and specialization. It includes the presence and type of symmetry in the body plan, the degree of development in sensory organs and brain, the presence and type of body cavity, the presence of body segmentation, and the structure of the digestive system. The members of the latest phylum, including vertebrate animals such as seals, whales, horses, and humans, also exhibit a trend toward increasing the size and sophistication of the brain.

Based on these traits, animals can be grouped into approximately thirty-one phyla. The nine major phyla include, from simple to more complex, Porifera (sponges), Cnidaria (hydra, anemones, and jellyfish), Platyhelminthes (flatworms), Nematoda (roundworms), Annelida (segmented worms), Arthropoda (insects, arachnids, and crustaceans), Mollusca (snails, clams, and squid), Echinodermata (sea stars and sea urchins), and Chordata (primarily vertebrates). Arthropoda is the largest phylum, with nearly four million species.

The Sponges, Hydra, Anemones, and Jellyfish

Sponges (phylum Porifera) are the simplest multicellular animals that lack true tissues and organs. They resemble colonies in which single-celled organisms live together for mutual benefit. However, individual sponge cells are able to survive and function independently. All sponges, whether single-celled or colony-like, have a similar body plan. The body is perforated by numerous tiny pores, through which water enters, and by fewer large holes, through which water is expelled. Water travels within the sponge through canals where oxygen is extracted, and microorganisms are filtered into cells for digestion. Some sponges can grow more than a meter in height. More than 9,600 species of sponges have been identified, all of which are aquatic, and most are marine.

The phylum Cnidaria is composed of hydroids, anemones, corals, marine parasites, and jellyfish. More complex than sponges, cnidarians have distinct tissues, including contractile tissue that acts like muscle and nerve net that spreads through the body and controls movement and feeding behavior. However, they lack true organs and a brain. Their beautiful and diverse body shapes are variations of two basic body plans: tentacled and jellyfish-like. Tentacles attach to rocks and reach upward for grasping, stinging, and immobilizing prey. A jellyfish-like body can easily be carried by ocean currents. Cnidarians are radially symmetrical, with body parts arranged in a circle around the mouth and digestive cavity. All cnidarians are predators, but none hunt actively. They rely upon their tentacles to grasp small animals floundering by chance into contact with them. Once stimulated by contact, special cells called cnidocytes explosively inject poisonous or sticky darts into prey. The immobilized prey is forced through an elastic mouth into a digestive sac. The undigested food is expelled through the mouth. Cnidarians may reproduce sexually or asexually. Of the 11,000 or more species in this phylum, all are aquatic, and most are marine. One of these, the corals in the class Anthozoa, is of particular ecological importance. They protect coastlines, support local economies, supply food, and have emerging medicinal applications. Coral reefs are among Earth's oldest and most ecologically diverse areas, but climate change is causing them to turn white and die, which is called mass bleaching.

Diverse Forms of Worms

Flatworms (phylum Platyhelminthes) are more complex than cnidarians, yet are the simplest organisms with well-developed organs. Their bilaterally symmetrical bodies are an adaptation to active movement, as found in other, more complex organisms. Their sense organs, consisting of light-detecting eyespots and cells responsive to chemical and tactile stimuli, inform their bodies whether to feed, forge onward, or retreat. When the flatworm encounters smaller animals, it sucks its prey through a muscular tube called the pharynx, located in the middle of the body. Compared with more complex organisms, however, flatworms lack both respiratory and circulatory systems. They can produce sexually or asexually. Most flatworms are hermaphroditic, possessing both male and female sex organs within one body. Examples of flatworms include parasitic tapeworms and flukes.

Roundworms (phylum Nematoda) reside in nearly every habitat on Earth. Though many more species likely exist, 28,000 had been named in the mid-2020s. They are largely microscopic, although some may reach a meter in length. They have a rather simple body plan, with a tubular gut that runs from mouth to anus. A fluid-filled hydrostatic skeleton provides support and a framework against which muscles can act. They also have a tough but flexible cuticle on the outside of the body and a simple brain that processes and transmits information. They do not have circulatory or respiratory systems. Most nematodes reproduce sexually, with the male fertilizing the female by injecting sperm inside her body. Nematodes play a crucial role in breaking down organic matter in ecosystems. Some are also parasites to humans or other animals, such as hookworms that infect human feet, Trichinella worms that cause trichinosis, and heartworms that attack dogs’ hearts.

The prominent feature of the phylum Annelida is the segmentation of the body into a series of repeating units; hence, they are called segmented worms. Each body compartment is controlled by separate muscles, collectively capable of far greater complexity of movement than in other worms. A well-developed closed circulatory system distributes gases and nutrients throughout the body. Primitive hearts, in essence short, expanded segments of specialized blood vessels, can contract rhythmically. A simple brain located in the head plus nerve cords along the length of the body and within each segment control movement and other activities. Among the 22,000 or so species identified, the best-known examples are the earthworm and its relatives, ragworms and leeches. However, the largest annelids, the polychaetes, live primarily in the ocean.

The Arthropods, Molluscs, and Echinoderms

The phylum Arthropoda comprises insects, spiders, and crustaceans. By any standard, whether a number of individuals or a number of species, arthropods are the most dominant animals on Earth. A mere 10 percent of animals described in this phylum constitute one million species, including insects (class Insecta), spiders and their relatives (class Arachnida), and crabs, shrimp, and their relatives (class Crustacea). The enormous success of arthropods is due to several adaptational features. The exoskeleton allows precision movement; segmentation generates specialized and more effective organ systems; these, in turn, allow higher efficiency in gas exchange, circulation, and information processing. Most arthropods have well-developed sensory systems, including compound eyes and acute chemical and tactile senses. Of the three classes, insects are the most diverse and abundant, accounting for over one million species identified. Insects usually have three pairs of legs plus two pairs of wings. Their ability to fly helps them escape from predators and find widely dispersed food. Insects normally go through radical changes in body form through metamorphosis, from egg to larva to pupa and finally to winged adults that mate and lay eggs.

Spiders and scorpions are examples of the class Arachnida. They typically have eight walking legs and are mostly carnivores, living on either a liquid diet of blood (ticks and mosquitoes) or predigested prey (scorpions). Simple eyes equipped with a single lens are extremely sensitive to movement, which helps in catching prey or escaping from predators. There are about 110,000 species of arachnids. Crab, shrimp, crayfish, and their relatives make up the class Crustacea, comprising between fifty and sixty-seven thousand species. They are largely aquatic, with a wide variation in size. Except for two pairs of sensory antennae and mostly compound eyes, they are highly variable in body form.

As their name suggests, members of the phylum Mollusca—snails, clams, and squid—have a moist, muscular body supported by a hydrostatic skeleton. Some have a shell of calcium carbonate to protect their body; others escape predation by moving swiftly or by being distasteful if caught. They have an open circulatory system. Their nerve systems are more advanced than those of arthropods in that more nerves are concentrated in the brain. Reproduction is sexual; some species have separate sexes, and others are hermaphroditic. Together, there are over 100,000 species identified, among which, clams, octopuses, oysters, scallops, snails, and squid are the most familiar.

Sea stars, sea urchins, and sea cucumbers compose the phylum Echinodermata. These animals are mostly marine, and adults have radial symmetry and lack a head and distinct brain. They have very simple nervous systems and hence move very slowly on numerous, tiny tube feet. They feed on algae or small particles sifted from sand or water. Most species reproduce by releasing sperm and eggs into the water, where larvae develop upon fertilization. Another distinct feature of echinoderms is their endoskeleton, a hard shell of calcium carbonate enclosed by an outer skin.

Phylum Chordata: The Tunicates, Lancelets, and Vertebrates

Animals of this phylum exhibit tremendous diversity in form and size. They include small sea squirts and lancelets (invertebrates), and birds, fish, amphibians, reptiles, and mammals (vertebrates). Members of this phylum possess four characteristics at some stage of their lives: a notochord—a stiff yet flexible rod that extends the length of the body and provides an attachment site for muscles; a dorsal, hollow nerve cord at the anterior end of the notochord that becomes a brain; specialized respiratory openings called pharyngeal gill slits; and a tail that extends past the anus. There are only two classes of invertebrates in Chordata, lancelets and tunicates, both of which are small marine animals. Lancelets reside mainly in the sandy sea bottom and live by filtering tiny food particles from the water. Sea squirts, a member of the tunicates, send out a forceful jet of water in response to touch or danger. Their filter-feeding, saclike bodies move slowly via contraction.

Vertebrates are the most conspicuous animals on Earth. Their backbones and other adaptations have contributed to their success. There are five major classes of vertebrates.

Jawless fishes (Agnatha) were the earliest vertebrates in the sea. Two examples are hagfishes and lampreys. The colorful hagfishes are strictly marine, living in communal burrows in mud, feeding on polychaete worms. Lampreys live in both fresh and saltwater. Some lampreys are parasitic, attaching to fish with suckerlike mouths lined with rasping teeth. They live on blood and body fluids sucked from their hosts. Cartilaginous fishes (Chondrichthyes) are skillful predators, and include sharks, skates, and rays. Their skeletons are made up exclusively of cartilage, void of bone. Many shark species have several rows of razor-sharp teeth, with back rows moving forward as front teeth are lost to action or aging. Most sharks, as most skates and rays, are shy and retiring creatures that do not attack humans. A few species, however, can be deadly when irritated. Bony fishes (Osteichthyes), spread over a wide range of aquatic habitats, are the most diverse and abundant vertebrates on Earth. As suggested by their name, bones rather than cartilage make up their skeletons. Of over 17,000 species identified, all bony fishes have bladders that help them float effortlessly. Some have lungs and modified fins that work as legs, which help them to survive periodic drying in freshwater habitats.

Amphibians (Amphibia) live a double life between aquatic and terrestrial habitats. They represent the transition of life from water to land. Some adaptations, such as lungs, a three-chambered heart, and moist skin, help them live a temporary land life. However, other traits, requiring water for fertilization and juvenile development, restrict the range of amphibian habitats on land. Their double life and permeable skin have made amphibians particularly vulnerable to pollutants and environmental fouling. Over 8,700 species have been identified, including frogs, toads, and salamanders. The seven thousand species of reptiles (Reptilia) identified have bodies of diverse forms. Turtles, snakes, lizards, alligators, and crocodiles are all reptiles, as well as the huge and now-extinct dinosaurs. Reptiles have more efficient lungs than amphibians, a tough, scaly skin that resists water loss and protects the body, a mechanism of internal fertilization, and a shelled egg. The diversity of birds (Aves) is revealed through 10,000 to 11,500 species, including the delicate hummingbird, the endangered spotted owl, and the largest bird, the ostrich. Their ability to soar gracefully in the air depends on many anatomical and physiological traits. These features include a light body with hollow bones, light wings with feathers that also provide protection and insulation, reduced reproductive organs during nonbreeding periods, a single ovary in female birds, acute eyesight, and a delicate nervous system that facilitates the extraordinary coordination and balance needed for flight. Birds, which are warm-blooded, also have four-chambered hearts that help to maintain high body temperature and a high metabolic rate, crucial for flight. The last vertebrate class, mammals (Mammalia), is represented by some 6,400 species. In addition to being warm-blooded with high metabolic rates, mammals normally possess hair, produce milk for their offspring, assume a remarkable diversity in form, and possess more highly developed brains than any other class. The bat, cheetah, elephant, mole, monkey, seal, and whale exemplify the radiation of mammals into nearly all habitats, with their bodies finely adapted to their lifestyles.

Principal Terms

Class: The taxonomic category composed of related genera; closely related classes form a phylum or division

Invertebrates: Animals lacking a backbone

Phylogeny: The evolutionary history of a group of species

Phylum: The taxonomic category of animals and animal-like protists that is contained within a kingdom and consists of related classes

Species: A group of animals capable of interbreeding under normal natural conditions; the smallest major taxonomic category

Taxonomy: The science by which organisms are classified into hierarchically arranged categories that reflect their evolutionary relationship

Vertebrates: Animals with a backbone or vertebral column

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