Invertebrate

I

Introduction

Invertebrate, any animal lacking a backbone. Invertebrates are by far the most numerous animals on Earth. Nearly 2 million species have been identified to date. These 2 million species make up about 98 percent of all the animals identified in the entire animal kingdom. Some scientists believe that the true number of invertebrate species may be as high as 100 million and that the work of identifying and classifying invertebrate life has only just begun.

Invertebrates live in a vast range of habitats, from forests and deserts to caves and seabed mud. In oceans and lakes they form part of the plankton—an immense array of miniature living organisms that drift in the surface currents. Invertebrates are also found in the soil beneath our feet and in the air above our heads. Some are powerful fliers, using wings to propel themselves, but others, particularly the smallest invertebrates, float on the slightest breeze. These tiny invertebrates form clouds of aerial plankton that drift unseen through the skies.

Although the majority of invertebrates are small, a few reach impressive sizes. The true heavyweights of the invertebrate world are giant squid, which can be over 18 m (60 ft) long and can weigh more than 2,000 kg (4,000 lb). The longest are ribbon worms, also known as nemerteans, whose pencil-thin bodies can grow up to 55 m (180 ft) from head to tail. At the other end of the size scale, animals called rotifers rank among the smallest invertebrates of all. Some species may reach 3 mm (0.12 in) in size, but most are less than 0.001 mm (0.00004 in), smaller than the largest bacteria.

II

Physical Characteristics

Due to their numbers and variety, invertebrates share only a single trait in common: the absence of a backbone. Many invertebrates have no hard body parts at all. These soft-bodied invertebrates, which include earthworms, keep their shape by maintaining an internal pressure, similar to the air pressure within an inflated balloon. However, having a soft body has disadvantages, one of which is that it leaves animals vulnerable to attack from predators.

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To defend against predators, other invertebrates have evolved exoskeletons, hard outer coverings such as the shells found in clams and mussels and the body cases that surround adult insects. As well as protecting the animal, these exoskeletons also provide anchorage for muscles. On land, a body case is also useful because it prevents the water that bathes internal structures from evaporating. As a result the animal does not dry up and die. Arthropods, animals with a hard, outer skeleton and a jointed body and limbs, make up the single largest group of invertebrates. Arthropods include insects, crustaceans, and arachnids, such as spiders and ticks.

Invertebrates have two basic body plans. Some invertebrates, such as corals and sea anemones, have a circular body plan arranged around a central mouth, similar to the way spokes radiate out from the hub of a wheel. This type of body plan is known as radial symmetry. Animals with radial symmetry often spend their adult lives fastened in one place, like the sea anemone that attaches to a rock, waiting for food to pass by. By contrast, invertebrates that move in search of food, such as flatworms, have an elongated body plan known as bilateral symmetry. Invertebrates with bilateral symmetry have right and left halves that mirror each other, and they typically have a definite front and back end. They have a head that often contains one or more pairs of eyes, together with organs that can taste, smell, or touch. However, major sense organs are often found on other body parts among some invertebrates. Katydids, for example, have hearing organs on their front legs, just below kneelike joints.

Compared to vertebrates (animals with backbones), most invertebrates have simple nervous systems, and they behave almost entirely by instinct. This system works well most of the time, even though these animals cannot learn from their mistakes. Moths, for example, repeatedly flutter around bright lights, even at the risk of getting burned. Notable exceptions are octopuses and their close relatives, which are thought to be the most intelligent animals in the invertebrate world. Studies have shown that these animals have the ability to learn. In some experiments they have solved simple puzzles, such as opening containers to retrieve food.

Invertebrates differ from each other internally in a wide variety of ways. Some have respiratory organs, circulatory systems, and excretory organs for getting rid of waste. The simplest invertebrates, such as placozoans, survive with few or no specialized organs at all. These animals absorb what they need from their surroundings—a way of life that works only in watery habitats and only with small animals.

III

Types of Invertebrates

Zoologists (scientists who study animals) classify invertebrates into about 30 major groups, known as phyla. These phyla vary enormously in the number of species they contain. Arthropods (phylum Arthropoda) are the invertebrate phylum with the most species—more than one million known species and countless more awaiting discovery. The mollusks (phylum Mollusca) make up the second largest group of invertebrates, with at least 50,000 species. Among the simplest invertebrates are the sponges (phylum Porifera). Other major invertebrate phyla include the cnidarians (phylum Cnidaria), echinoderms (phylum Echinodermata), and several different groups of worms, including flatworms (phylum Platyhelminthes), roundworms (phylum Nematoda), and annelids (phylum Annelida).

Arthropods live in every habitat on Earth from mountaintops to hydrothermal vents, springs of hot water located on the deep ocean floor. Surrounded by protective exoskeletons, arthropods have tubular legs that bend at flexible joints. This unique characteristic sets them apart from all other invertebrates, and it enables them to hop, walk, and run.

Insects dominate the arthropod phylum. Making up 90 percent of all arthropods, insects have a strong claim to be the most successful animals in the world. On land, they live in almost every habitat, aided by their small size and, for many, their ability to fly. They also live in fresh water, but remarkably, they have failed to colonize the sea. Some zoologists believe this is because crustaceans have already exploited this habitat to its fullest.

Mollusks make up the second largest group of invertebrates. Even by invertebrate standards mollusks are extremely varied. Mollusks include snails, clams, octopuses, and squid, as well as some lesser-known animals, such as chitons and monoplacophorans. Some mollusks, such as bivalves, are sedentary animals, while others such as squid are jet-propelled predators that are the swiftest swimmers in the invertebrate world. Most sedentary mollusks are filter-feeders—that is, they feed on tiny organisms that they strain from water. Other mollusks, including snails and other gastropods, scrape up their food using a radula—a ribbonlike mouthpart that is unique to mollusks and covered with rows of microscopic teeth.

Sponges have many unique characteristics that set them apart from other kinds of animal life. They are the only animals with skeletons made of microscopic mineral spikes and the only ones that feed by pumping water through hollow pores. Some of their cells are remarkably like free-living protozoans called collar flagellates. To evolutionary biologists, this resemblance strongly suggests that sponges and other invertebrates arose from protozoan-like ancestors.

Cnidarians include jellyfish, sea anemones, and corals. Their bodies have two layers of cells, a central digestive cavity, and a mouth surrounded by stinging tentacles. Most cnidarians are quite small, but the largest jellyfish—a species from the North Atlantic Ocean—can grow over 2 m (7 ft) across, with tentacles over 30 m (100 ft) long.

Among the major phyla, the echinoderms are the most distinctive and unusually shaped. They include starfish, sea urchins, and sea cucumbers and are the only animals with a five-pointed design. They live in the sea and move with the help of tiny fluid-filled feet—another feature found nowhere else in the animal world.

Zoologists recognize several different groups of worms. The phylum known as flatworms contains the simplest animals possessing heads. Nerves and sense organs are concentrated in the head. Most flatworms are paper-thin and live in a variety of wet or damp habitats, including the digestive systems of other animals. Roundworms represent another phylum. They are more complex than flatworms, with cylindrical bodies and mouthparts designed to pierce their food. Although flatworms have digestive systems with only one opening, the roundworm digestive system runs from the mouth straight through its body to an excretory opening—a body plan shared by more advanced invertebrates as well as vertebrates.

Although roundworms are extremely abundant, they often go unseen. So, too, do many worms that live exclusively in the sea, such as spoonworms (phylum Echiura), peanut worms (phylum Sipuncula), and pogonophores (phylum Pogonophora). Annelids are a large group of worms that contain some more familiar species. Among them are earthworms—annelids that feed by burrowing through the soil. An earthworm’s body is divided into repeated segments or rings, a feature shared by annelids as a whole.

IV

Reproduction and Life Cycle

Invertebrates display a wide variety of methods of reproduction. Some invertebrates reproduce by asexual reproduction, in which all offspring are genetically identical to the parent. Asexual reproduction methods include fragmentation, in which animals divide into two or more offspring, and budding, in which animals sprout buds that break away to take up life on their own. The majority of invertebrates reproduce sexually. The genes from two parents recombine to produce genetically unique individuals. For most invertebrates, sexual reproduction involves laying eggs. With a few exceptions, such as scorpions and spiders, most invertebrates abandon their eggs as soon as they are laid, leaving them to develop on their own.

When invertebrate eggs hatch, the animals that emerge often look nothing like their parents. Some are so different that, in the past, zoologists mistook them for entirely new species. Young like this are known as larvae. As they grow up, larvae change shape, a process known as metamorphosis. A larval stage enables invertebrates to live in different habitats at different stages of their lives. For example, adult mussels live fastened to rocks, but their larvae live floating among plankton. By having larvae that drift with the currents, mussels are able to disperse and find homes with new food sources for their adult life.

The change from larva to adult is quite gradual in many invertebrates, such as crabs and lobsters, but in insects it can be much more abrupt. Caterpillars, the larvae of butterflies and moths, often live for several months, but they take just a few days to turn into adults. During the transition stage, known as the pupa, the caterpillar’s body is broken down and reassembled, forming an adult insect that is ready to breed.

Most invertebrates are short-lived animals, but slow-growing species often break this rule. Wood-boring beetles can live well into their teens, while queen termites can live 40 years or more. But in the invertebrate world, the real veterans live in the sea. Growth lines on bivalve shells suggest that some clams can live to be 400 years old or more. An age of about 200 years has been claimed for pogonophoran worms living around hydrothermal vents in the darkness of the deep seafloor.