{in-vurt'-i-brayt} The invertebrate group is one of the two general categories of animals. The other group, vertebrates, includes those animals having backbones composed of a series of articulating vertebrae (fishes, amphibians, reptiles, birds, and mammals). Invertebrates lack vertebrae and include the remainder of the animal kingdom. The invertebrate group covers a wide range of organisms, from simple single-celled protozoans to those members of the phylum Chordata that lack a vertebral column. As such it is also an artificial category, splitting the phylum Chordata and including all the other phyla. Thus, invertebrates constitute almost the entire animal kingdom. Still, it is a useful category and unites a diverse assemblage of organisms that may be studied by a common approach distinct from the study of vertebrates. HABITAT In terms of habitat, distribution, and abundance, invertebrates are highly successful organisms. They occupy all habitats; the deepest samples from the deep-sea trenches include invertebrate animals. They are found in all types of marine substrates, from soft oozes to rocky bottoms. Swimming forms may be found at all depths of the sea and include forms specialized to live in the nutrient-poor, perpetually cold and sunless waters of the deep sea as well as a species specialized to live under the surface film of the open ocean. Invertebrates occur over a wide range of saline habitats, being common in fresh, brackish, and fully marine environments. Some specialized forms may also occur in hypersaline environments, where the salinity may greatly exceed full-strength seawater. These habitats occur in isolated lagoons along tropical coasts and pools high in the intertidal zone, where evaporation gradually increases their salinity. Terrestrial habitats, including subterranean locations, are universally occupied by invertebrate forms. Even the air has an invertebrate fauna in insects and spiders. Some tiny spiders have been captured in high-altitude balloons. In all of these habitats invertebrates may occupy a wide range of temperatures, from near 0 deg C (32 deg F) in the ocean depths to a few forms living in hot springs exceeding 55 deg C (131 deg F). Invertebrates also range over a wide series of oxygen conditions. Some are limited to richly oxygenated freshwater riffles while others may only be found in oxygen-free parasitic environments or oxygen-poor black sulfide muds. Invertebrates also include many forms that live on or in other organisms. This category includes parasites that harm the host; commensals, which do not affect their host; and mutuals, which improve the condition of their host. In all these habitats, invertebrate populations may reach astounding densities. Soil arthropods often attain densities of a half-million per square meter. The number of invertebrate species extant today is difficult to estimate. As many as 4.5 million species may exist, but only half have been named. Indeed, through habitat destruction, especially in the tropics, terrestrial species may be vanishing forever at a more rapid rate than scientists are discovering and describing them. The groups containing the largest number of species include the protozoans, roundworms, mollusks, and arthropods. All are estimated to have at least 100,000 species with only a fraction having been described. Among the arthropods, beetle species alone number at least 375,000. FOSSIL RECORDS Most invertebrate groups have ancient origins, and many have left a fossil record in old rocks. The study of the invertebrate fossil record is known as invertebrate paleontology. The fossil record provides information of two kinds. Invertebrates with hard skeletal structures have left remains or imprints of them in the rocks by a process known as fossilization. A more elusive record has been left by many soft-bodied, burrowing animals in the form of trace fossils, which are the remains of burrows, tubes, and track imprints in soft substrates that were gradually mineralized. The oldest invertebrate fossils date to the Precambrian Period of the Proterozoic geological epoch more than 530 million years ago. Fossils remaining from this period are few, and little is known of these life forms. Recent discoveries include a fossil clam believed to be more than 720 million years old. Fossil representatives of all major phyla with significant skeletal features have been found in the Cambrian Period (570-500 million years ago). The difference between the Precambrian and Cambrian periods may represent some unknown major event that led to the evolution of hardened skeletal structures for rapid movement (muscle attachments) or defense (armor). Controversy exists over the so-called \Tediacaran fauna\t of the late Precambrian, which many theorists consider an evolutionary dead end rather than representing a transitional stage leading to forms existing today. As geological time passed, certain organisms that are extinct, or are a minor component today, flourished. The trilobites (related to modern horseshoe crabs) were dominant members of the Ordovician (500-425 million years ago) and Silurian Period (425-400 million years ago) faunas but then declined and were extinct by the end of the Permian Period (280-230 million years ago). Nautiloids, shelled squidlike mollusks, were an important component of these early periods. Today only the beautiful coiled Nautilus still exists. Some groups endured as faunal dominants for very long periods only to decline at a later date. The lamp shells (brachiopods) became abundant in the Ordovician Period and were a major marine group until the Paleocene Epoch of the Tertiary Period (65-55 million years ago). About 260 species of lamp shells remain today as compared with at least 12,000 described fossil forms. Although most groups are as ancient as the Cambrian, the first traces of a few groups do not appear in the fossil record until a much later date. Primitive crustaceans are first seen in the Ordovician. Scorpions become the first air-breathing terrestrial animals by the Silurian, preceding the first vertebrate onto the uninhabited soil by about 50 million years. Primitive insects occur in the Devonian. Insects and arachnids (spiders, scorpions, and related groups) have greatly increased and continue to be important today. Through examination of the fossil record we are able to obtain a glimpse into the life of the past. From fragmentary remains, paleontologists are able to describe the anatomy and even the ecology and behavior of these extinct forms, making use of the tantalizing fragments of evidence at their disposal. Description of the invertebrate fossil assemblages also has considerable economic importance as these associations and arrangements help geologists determine the age and type of different geological strata. These cues indicate, among other things, the oil-bearing potential of different regions. Since the invertebrate group contains virtually the entire animal kingdom, it is not surprising that it includes bewilderingly diverse kinds of organisms. The following discussion proceeds generally from simple and primitive forms to more highly evolved types. CLASSIFICATION Protozoa \TProtozoa\t are single-celled organisms. These individual cells are structurally much more complex, however, than any given cell in a multicellular organism. Protozoans are a common (30,000 described species), large group found in marine, freshwater, and terrestrial habitats and include many parasitic representatives, among them the causative agent of malaria, one of the most important human diseases. Mesozoa \Lmesozoan\ls are a small group (about 50 species) and entirely parasitic, one branch of which occurs only in the kidneys of octopuses and squids. These parasites consist of a small number of cells of merely a few cell types. Some zoologists regard them as genuinely primitive, whereas others classify them as degenerate flatworms. Porifera \Lsponge\ls (5,000 species) are simple, attached filter-feeding organisms. They are common in marine habitats; a few species are found in fresh water. Coelenterata \Lcoelenterate\ls (10,000 species) include the jellyfishes, hydroids, sea anemones, and colonial corals. All are marine except for a few freshwater forms (including the hydra, the common pond dweller). The \Tcomb jelly\t (phylum Ctenophora, 80 species) is closely related to the coelenterates. All are marine, most being pelagic (deep-sea dwelling). Platyhelminthes \Lflatworm\ls (nearly 13,000 species) are found in all habitats. This group includes the common planarian used extensively in the classroom and laboratory, as well as important parasitic groups, the flukes and tapeworms. Closely related are the \Lribbon worm\ls (phylum Nemertinea, 600 species), a mostly marine group with a very few freshwater, terrestrial, and parasitic members. Pseudocoelomata The pseudocoelomate phyla are mostly wormlike and are grouped together because they possess a few developmental similarities. \Lspiny-headed worm\ls (phylum Acanthocephala, 300 species) are all parasitic. The adult worms are attached to vertebrate intestines. Life cycles are complex, involving two host species. The rotifers (1,500 species) are small freshwater organisms; a few are marine and parasitic. Gastrotrichs (phylum Gastrotricha, 150 species) are similar but are covered with bristles. They are inhabitants of fresh and marine water. The kinorhynchs (64 species) are all marine and are common in interstitial habitats, living in the fluid between the grains of mud or sand. The gnathostomulida (phylum \TGnathostomata\t, 90 described species) are small wormlike animals living in the oxygen-poor environment of marine sands in shallow water. This poorly known habitat probably accounts for the fact that they escaped scientific detection until the mid-20th century. The roundworms, or \Lnematode\ls (phylum Nematoda, 10,000 species described), are one of the most successful phyla. Parasitic members infect plants as well as every group of invertebrates and vertebrates (including human beings). Additionally, many are free-living marine, freshwater, and terrestrial types. With this great habitat diversity, nematodes are remarkable for their structural similarity. The gordian, or \Thorsehair worms\t (phylum Nematomorpha, 250 species), are free-living aquatic forms as adults, but the juveniles are parasitic in the body cavities of arthropods. Bryozoa \Lbryozoan\ls (4,000 species) are small, colonial, and mostly marine with a few freshwater representatives. They feed using a crescent-shaped crown of tentacles known as a lophophore. The small and structurally similar \Lentoproct\ls (phylum Entoprocta) are sometimes included with the bryozoans although they show embryological similarities to the pseudocoelomates. Two other phyla also feed with a lophophore. The \Lphoronid\ls are a small group (only 15 species), but they are often common animals in marine muds. Here, they live enclosed within a leathery or chitinous tube. The lamp shells, or \Lbrachiopod\ls (phylum Brachiopoda, 260 species) are entirely marine and mostly deep-water inhabitants. They resemble clams but are attached to rocks by a flexible stalk and possess a lophophore. Mollusca The \Lmollusk\ls are a major phylum (130,000 species), including the snails, bivalves (clams and oysters), chitons, cephalopods (squids and octopuses), as well as a few minor groups. Mollusks have invaded every major environment, being found in marine, freshwater (snails and bivalves only), and terrestrial (snails only) habitats. A few snails are also parasitic. Mollusks include some of the largest invertebrates (giant clam and giant squid) and probably the most intelligent (squid and octopuses). Annelida The \Tannelid\t worms (8,800 species) encompass many marine (polychaetes), freshwater, and terrestrial (earthworms and other oligochaetes) types and an important group of parasites (leeches). They all have many-segmented bodies and well-developed musculature. Three small phyla, all marine, are sometimes treated as annelid allies, the \Lspoonworm\ls (phylum Echiura, 60 species), \Lpeanut worm\ls (phylum Sipuncula, 275 species), and phylum Priapulida (8 species). All are burrowers, showing few remnants of their presumed former segmentation. The \Lbeardworm\ls, phylum Pogonophora (80 species), are very elongate and dwell in deep-water sediments. Recognized in the early 20th century, recent evidence suggests that they are merely a type of polychaete annelid. Onychophora The \Lonychophoran\ls (65 species) are a curious "missing link" between the two great phyla Annelida and Arthropoda. They are small forms living under leaf litter of tropical forests. Arthropoda The \Larthropod\ls (about 900,000 species) are enormously diverse. Recent work suggests that some of the major branches are not closely related and should be relegated to separate phyla. Arthropods include the chelicerates (trilobites, horseshoe crabs, spiders, scorpions, ticks, and mites). Present-day representatives are mostly terrestrial with ticks and mites representing important parasitic types. The crustaceans (fairy shrimp, ostracods, copepods, barnacles, shrimp, crabs, lobsters, pillbugs) are mostly marine, but also include important freshwater, terrestrial, and parasitic representatives. Millipeds and centipedes have only terrestrial representatives. The insects, however, include important freshwater and parasitic groups and a few marine forms. Insects dominate the terrestrial fauna. Among the parasitic insects are lice and fleas as well as a large group (more than 100,000 species) of wasps that are parasitoids of other insects. These parasitoid wasps look like parasites but they slowly devour the host and thus have a predatory character. Insects are also remarkable for their development of highly organized social colonies among the termites, ants, bees, and wasps. Chaetagnatha The \Larrowworm\ls (55 species) are a small group of transparent, unsegmented, pelagic predators. They feed on other marine planktonic animals. Echinodermata The \Lechinoderm\ls (5,500 species) are the only major phylum limited solely to marine conditions. Included here are the sea lilies, sea cucumbers, sea urchins, sand dollars, starfish, and brittle stars. Acorn worms, or \Lhemichordate\ls (100 species), are also all marine. Most burrow in soft substrates. Chordata The final, partially invertebrate phylum encompasses three groups of seemingly dissimilar \Lchordate\ls. Two are considered invertebrates, and the third and by far the largest is the vertebrates. All possess three distinctive features at some stage in their lives: a flexible stiffening notochord, a dorsal hollow nerve cord, and gill clefts. Of the two invertebrate groups, the urochordates (class Tunicata, 1,300 species) include the abundant, attached, filter-feeding sea squirts (tunicates) as well as some less well-known planktonic forms. Like the urochordates, the cephalochordates are entirely marine. Cephalochordates, however, are active swimmers, filter feeding while moving in and out of the sand along beaches in tropical and subtropical regions. STRUCTURE AND FUNCTION Because they are such a diverse group of organisms, many different schemes have been advanced to classify the invertebrates. Important features considered with regard to the greater complexity of the body plan are changes in the basic symmetry of the organism, its gut structure, the development of body cavities, and the increase in cell types and their arrangement in organ systems. Body Plan The cells of sponges and mesozoans are differentiated into a few cell types, and these tend to be organized into tissues, groups of similar cells having a coordinated function. There is no gut in these organisms, although in sponges water does move through a series of channels. The coelenterates show the next series of advances. Here, symmetry is radial (biradial in the comb jellies), a gut is present with a mouth but usually lacks an anal opening, and tissues are grouped into primitive organ systems such as the digestive system. All of the tissues tend to be arranged in outer and inner layers only. In the flatworms the interior is filled with a solid mass of tissues and cells. Muscles tend to be extensively developed, and symmetry is now bilateral. With bilateral symmetry, the flatworms show the first signs of increasing cephalization, the tendency to group the sense organs and feeding structures at the anterior, or head, end. So situated, these features are now in a position to meet the environment. Ribbon worms are similar to flatworms but the gut is complete, with an anus. The pseudocoelomates have developed the first primitive body cavity. It is not lined with a sheet of cells, like a peritoneum, and is thus not a true body cavity, or coelom. These phyla also exhibit well-developed and complete digestive tracts. Among the unique features of some pseudocoelomate groups is cell constancy, in which certain organs invariably consist of a constant number of cells. The remaining phyla all have a coelom; although in some groups it is secondarily reduced. Because of the development of a coelomic body cavity, the higher invertebrates are able to attain life-styles possible to only a limited extent in the primitive phyla. Here we see many strong swimmers and vigorous burrowers. Based on details of embryological development the higher invertebrates are grouped into two main branches: the protostomes and the deuterostomes. In protostomes the mouth develops from the blastopore in the embryonic gut. In deuterostomes, the mouth develops an opening other than the blastopore, which becomes the anus. At the base of this separation lie the lophophorates, united in having a lophophore and sharing characteristics of both main branches. Protostomes include the annelids, annelid allies, mollusks, and arthropods, all closely related. The mollusks appear to have developed from a presumed annelid ancestor before segmentation (the serially repeated arrangement of many organs, typical of the annelids) was well developed. Segmentation allows for subsequent specialization of different segments for different functions: sensory perception, food capture, and respiratory, locomotive, and reproductive activities. Among the arthropods, this specialization is carried to an extreme. Certain segments are fused together forming complex structures such as a head or an abdomen. The arthropod body cavity is greatly reduced. Only small cavities associated with the gonads, kidneys, and heart remain. It is replaced by a hemocoel, another type of functional body cavity filled with blood. A further arthropodan elaboration is the sturdy exoskeleton, flexible and chitinized in most of the land-dwelling forms, sturdy and calcified in many marine arthropods. With this skeleton, arthropods also possess a series of jointed appendages. As do body segments, these appendages tend to serve specialized functions as mouth parts, walking legs, and swimming appendages. Mollusks also show a reduction of their coelom in a manner similar to arthropods but have only traces of segmentation. In most groups a heavily calcified shell protects these animals from predators and environmental stress. In cephalopod mollusks (squids and octopuses) the shell is greatly reduced or absent. Cephalopods have highly developed muscular and nervous systems and rely on speed, agility, and intelligence instead. The deuterostomes include the arrow worms, echinoderms, acorn worms, and the lower chordates. Relations here are obscure except that the last two groups appear to be more closely related. The vertebrates, being chordates, also appear on this branch. Echinoderms present an unusual body plan having reverted to radial symmetry. Certain echinoderm organ systems are without counterparts elsewhere in the animal kingdom. It has been suggested that if any of the phyla come from outer space, it is the echinoderms. Hemichordates are closely related to the chordates. Both have a specialized anterior chamber with gill slits serving the dual functions of filter feeding and respiration. The body plan of certain parasitic forms differs greatly from that of their closest free-living relatives. Tapeworms and spiny-headed worms entirely lack a digestive tract. Food is absorbed through the general body surface. Parasites also tend to have prominent holdfasts, arrangements of hooks and suckers, to adhere to the host. As a result of the difficulty in locating hosts, parasitic forms often have outstanding reproductive capacities compared with related free-living forms. Body Covering The great structural diversity among invertebrates permits only a highlighting of common features here. Body surfaces may either consist of a delicate tissue layer or be covered with a tough and sometimes hard skeleton (for example, bryozoans, lamp shells, some annelids, mollusks, arthropods, and echinoderms). Only among the annelids and arthropods do we find terrestrial representatives; their external skeleton helps to prevent water loss, the chief stress produced by land environments. The integument of many invertebrates is often brilliantly colored, either providing camouflage or sometimes serving as a warning that the animal may be noxious. In the crustaceans, pigment granules occur in specialized cells, where they may be dispersed or contracted to permit gradual color change. In the cephalopod mollusks, pigment granules are found in little sacs operated by tiny sets of muscles. Here, color change can be instantaneous. Bioluminescence Representatives of several invertebrate phyla possess a capability for \Tbioluminescence\t. Organs located in the integument emit cold light and are used for species-recognition signals or camouflage. The luminescent substances of animals may be either contained within cells or produced as secretions from groups of cells. Some animals produce their own light, whereas others contain bacteria or fungi that actually synthesize luminescent substances. Circulatory System Circulatory systems are absent in small species of all invertebrate phyla, because substances can be moved throughout the body via simple diffusion. These forms have an open circulation system. Body-cavity fluids also serve this distributory function. Most higher invertebrates have at least a partially closed circulatory system where the fluid moves through well-defined blood vessels at least part of the time. In these animals, hearts, acting as muscular pumps, are necessary organs. A well-developed closed circulatory system is perhaps best seen in annelid worms. Mollusks and arthropods have a partially closed system with hearts and a few large blood vessels, but each organ is ultimately bathed directly in blood. Respiratory System Like circulatory systems, respiratory systems are also associated with large size. Oxygen can diffuse directly through the general body surface of small organisms. Large aquatic animals (mollusks, arthropods, annelids, annelid allies, and echinoderms) all have evolved gill-like structures with enormously expanded surface areas to increase the rate of gas exchange. These structures are closely associated with the circulatory system to distribute oxygen directly to cells and remove waste gases. Terrestrial organisms have evolved more protected respiratory structures since exposed gills would quickly dry up on land. Land snails, scorpions, and spiders have evolved lunglike structures, called book lungs, and insects have a unique tracheal system of passageways leading from openings in the body surface to the vicinity of every body cell. Excretory System Excretory systems are a third organ system that tends to be most elaborately developed in large animals; simple diffusion suffices for small types. An excretory system basically involving a filtering structure leading to a duct that removes the wastes is found in most phyla above the coelenterates. Nervous System The nervous systems of invertebrates show a series of advances paralleling the evolution of the major vertebrate groups. Sponges have only a general sensitivity and no specialized sense organs. The coelenterates have a network of specialized nervous cells but are still capable of only general responses. Simple light receptors and balancing organs are also seen in this group. Flatworms have a well-defined nerve ladder and show coordinated responses to stimuli. Eyes are better developed but still only distinguish light from dark. Good images are formed only in mollusks, arthropods, and a few annelids, as compared to other invertebrates. The cephalopod eye is a remarkable parallel development to the vertebrate eye. A cornea, iris, lens, and associated musculature permit clear binocular images to be formed on the light-sensitive retina and carried thence to the well-developed brain for sensory processing. In association with their excellent vision, the nervous systems of these mollusks show other important advances. Giant nerve cells, having a width of about 1 mm (0.04 in), run the length of the body. Rapid conduction along these cells initiates the animal's rapid escape response. Arthropods have a segmental nervous system with prominent nerve centers (fused ganglia) in each segment. Most crustaceans and insects possess compound eyes. Each eye is composed of many (sometimes hundreds) similar subunits. Each subunit is covered by a corneal layer and has a lens element and a few receptor cells. Light entering each of these subunits is prevented from passing to adjacent units by a series of pigmented cells between the subunits. Compound eyes seem to be particularly well suited to recognize rapid changes in movement, or detection of light and shade, rather than for the formation of images. In crustaceans the compound eyes are on movable stalks, and thus the visual field can be shifted or increased in size. Many invertebrates show a wide variety of touch receptors, chemical sensors, and balance detectors in addition to light receptors. Sound production and reception appears to be largely limited to insects and crustaceans. Echinoderms, while an advanced phylum in many respects, show only modest sensory capabilities related to their return to radial symmetry and a sedentary life-style. In addition to nervous conduction, messages may also be transmitted through slower systems involving hormonal effects. These are perhaps best seen in the insects and crustaceans where the cyclical shedding of the exoskeleton is induced by the increase in concentration of the substance, ecdysone, in the blood. Maturation in insects is controlled by the level of juvenile hormone. A decrease in juvenile hormone production leads to maturation with cessation of growth in the adult insect. Musculature Sea anemones and higher invertebrates all have well-developed musculature. For worm-shaped organisms to be capable of subtle movements, locomotory muscles must be arranged in at least circular and longitudinal bands. The curious roundworms lack circular musculature. The whiplike movements imparted by antagonistic bands of only longitudinal muscles enables this group to be readily identified. Reproductive System All organisms must reproduce, and thus reproductive systems are well-defined even at the coelenterate level. In addition to gonads, most invertebrates have a wide variety of accessory reproductive structures. Ducts and channels permit movement of reproductive products. Sperm may be stored in the male reproductive system in seminal vesicles or in the female system in seminal receptacles. Forms producing shelled eggs, including flatworms, mollusks, and arthropods, have accessory glands to produce the shells and a storage chamber (uterus) to hold the finished product. Although many groups have separate male and female individuals, most flatworms, some mollusks, and a few arthropods have both sexes combined in a single individual. Reproductive development of both sexes may be simultaneous or sequential with first the male and then the female system maturing. Sex may be determined genetically or, in a few forms, such as slipper shell snails, sex may be environmentally determined. Among several of the lower invertebrates the primary form of reproduction is asexual, involving the division of the parent individual into two or more daughter individuals. Regeneration Most invertebrate phyla have excellent abilities to regenerate parts of the body if injured. The freshwater planarian flatworm can regenerate if cut into several pieces. More limited abilities typify other phyla. For example, crabs and shrimp are able to regenerate their appendages, but they must wait for the next time their exoskeleton is shed for this process to be completed. Life Cycle Many invertebrates have complicated life cycles. These developmental stages are often specialized for different functions. Many marine forms release numerous eggs that hatch as planktonic larvae, specialized for dispersal. Larval stages are followed by a habitat-selection stage, in which the organism attempts to settle in a habitat suitable for the development of the juvenile and adult forms. Between successive stages, these animals typically undergo complicated metamorphoses. Virtually every tissue and organ of a stage may be reorganized to form a new structure in the succeeding stage. Phyla with particularly complicated life cycles include the coelenterates, parasitic flatworms, chordates, and some arthropods (insects and crustaceans). Armand Kuris Bibliography: Barnes, R.D., Invertebrate Zoology, 4th ed. (1980); Barrington, E.J., Invertebrate Structure and Function (1967); Barth, Robert H., and Broshears, Robert, The Invertebrate World (1983); Gardiner, M.S., The Biology of Invertebrates (1972); Hyman, L.H., The Invertebrates, 6 vols. (1940-67); Meglitsch, P.A., Invertebrate Zoology, 2d ed. (1972); Russell-Hunter, W.D., A Life of Invertebrates (1979); Wells, M., Lower Animals (1968).