What is an Invertebrate invertebrates invertebrates are animals without backbones While vertebrates have the same basic plan with a skull a jointed backbone extending into the tail and paired fore and hind limbs supported by bony girdles invertebrates have none of these things There are about 29 major groups of invertebrates that are all very different from the vertebrates and all quite different from each other Together they make up about 97 percent of the animal kingdom The remaining 3 percent of animals are the vertebrates fish birds mammals and so on Familiar invertebrates include snails clams earthworms flies wasps spiders and starfish but there are many others some of which are relatively obscure such as parasitic flukes and tapeworms Invertebrates play a large part in human health Some are responsible for transmitting diseases such as malaria that afflict hundreds of millions of people Although modern medicine has done much to reduce their effects malaria is once again a very serious problem in some parts of the world from which it had once been eliminated Invertebrates such as locusts aphids slugs and wireworms are crop pests Termites and wood-boring beetles can cause major structural problems in timber houses and destroy timber and paper products Clearly some invertebrates exert considerable pressures on humans while others are useful to people For example ants and earthworms do much to improve the quality of soils by burrowing and turning over the soil particles Bees produce honey while oysters and crayfish are eaten around the world Some invertebrates are the source of useful drugs and medicinal products for example the anticoagulants of leeches or the antiarthritis drugs taken from green mussels Others like many butterfly species are simply very beautiful to look at Insects are invertebrates and without them it would have been necessary for plant 4 life to have evolved different methods of pollination Invertebrate Diversity There is an astonishing range of invertebrate structures and lifestyles Adult sea-urchins for example are round and heavy with chalky skeletons They have no head they defend their bodies with spines and minute forceplike grooming organs and they move around on the seabed with hydraulic limbs Inside the mouth are five well-developed teeth that scrape plants off the rocks on which they live Their eggs develop into microscopic larvae that drift at the surface of the sea These larvae are unlike their parents in appearance and lifestyle They are not round or heavy and they have a right and left side But they turn into juvenile sea-urchins and sink to the bottom of the sea Compare the sea-urchin with a soft-bodied flatworm living amid the vegetation at the bottom of a pond Here is an animal that has a head with rudimentary eyes and smelling organs it has a mouth but no anus and its soft body ends in a tail The flatworm glides over the pond floor feeding on rotting vegetation and bacteria Its body is light and needs no hard skeleton for the muscles to pull on These two examples give us a glimpse of the range of animal architecture among invertebrates Compare them with the honey bee a flying insect that can communicate harvest pollen live socially and make honey The range of invertebrate forms and activity is huge Defining Characteristics Scientists have established a number of fundamental
WHAT IS AN INVERTEBRATE Mollusks share common structural features The phylum Mollusca includes sea hares 1 giant clams 2 octopuses 3 chetoderms 4 chitons 5 razor shells 6 elephant’s tusk shells 7 dog whelks 8 European whelks 9 and Neopilina limpetlike mollusks 10 1 2 3 Specialized jumper The desert locust above is an arthropod with a jointed exoskeleton 6 7 characteristics that help establish the classification and evolutionary relationships of invertebrates They include features such as symmetry Some animals such as sponges are asymmetrical having no particular shape Some such as sea anemones and corals are radially symmetrical having no head and with the mouth in the middle of a circular saclike body Some like worms and insects are bilaterally symmetrical having a head and with the body developed into left and right sides Other distinguishing characteristics include whether the cells of the body are organized into tissues and how many tissue layers make up the body The presence or absence and type of cavity inside the body are also considered important All these distinctions are rather technical But the result is that the invertebrates can 8 10 9 5 4 be arranged in an order of increasing evolutionary complexity in which the development of different types of body architecture can be understood and the relationships between phyla can be predicted The History of Invertebrates Although we believe that the first primitive land vertebrates appeared over 275 million years ago during the Carboniferous period the fossil record tells us that the invertebrates have been in existence for much longer probably about 650 million years We depend on the existence of fossils that can be dated in order to determine the longevity of animal groups Nowadays special techniques enable us to determine 5
the age of fossils with considerable accuracy The Paleozoic era stretched from 650 million years ago to 225 million years ago and it is divided into seven periods The earliest the Ediacarian 650 to 570 million years ago saw the appearance of the first animal fossils They are not easily assigned to any phyla we know today but resembled present day sea anemones worms and jointed limbed creatures By the end of the next period the Cambrian 570 to 500 million years ago invertebrates with hard skeletons had begun to appear They fossilized quite well and most of the phyla we recognize today were represented including trilobites a kind of crustacean brachiopods a type of mollusk and primitive echinoderms sea urchins By the end of the Ordovocian period 500 to 430 million years ago most of the classes of invertebrates we recognize today had established themselves and some such as the echinoderms and nautiloid mollusks had reached their peak of development The shorter Silurian period 430 to 395 million years ago saw the decline of some of the great marine groups but terrestrial invertebrates such as millipedes began to appear on land Arachnids and wingless insects next appeared on land in Anchor damage by small boats visiting coral reefs is just one example of thoughtless harm to the environment the Devonian period and winged insects developed in the Carboniferous period that ended 280 million years ago At this time there was a slow decline in the diversification of life forms and by the end of the Permian period 225 million years ago most groups had 7 1 5 2 3 dramatically declined in fact many had become extinct probably as a result of the cooling of the earth A slow recovery in invertebrate diversity began During the Jurassic period 192 to 135 million years ago most of the modern orders of animals became established and invertebrate classification would have started to 4 An Arctic seabed community during the Cretaceous period 135 to 64 million years ago Organisms included the belemnite 1 ammonite 2 sea anemone 3 gastropod 4 sea urchin 5 bivalve mollusk 6 and crab 7 6 6
look much the same as it does today Development declined in the Cretaceous period Coming to the Tertiary period beginning 64 million years ago most genera and families of invertebrates were established Why are Invertebrates at Risk Many invertebrates are small and some lack sophisticated means of controlling what is inside them Some the marine forms therefore take on an internal environment and salt and water balance that resembles seawater itself Others that live in fresh water may have only the crudest mechanisms to control their osmotic balance how much water and salt they have in their bodies and quite a few simple terrestrial forms cannot survive out of contact with moist soils Thus in many cases such species are unable to prevent their bodies taking in chemical pollutants or are unable to control the loss of moisture if the environment becomes starved of water For those invertebrates that are attached to the sea floor such as corals and sponges changes in seawater levels can be disastrous and changes in temperature cannot be overcome by migratory movements In contrast a number of other endangered invertebrate species such as butterflies and dragonflies are highly mobile but their life cycles may demand a particular type of vegetation or aquatic environment for their larvae to develop in Such environments are frequently damaged or destroyed by human activity especially the development of industries or urban areas The Situation Today Because invertebrates are relatively unknown and often lack the charisma of the large vertebrates such as mammals they do not gain the same amount of public support People are much more likely to contribute to conservation of the panda than an endangered beetle or spider species Furthermore it is difficult to give precise figures on the endangered status of the invertebrates There are probably over 40,000 species of vertebrate but no one knows how many species of invertebrate exist on earth Nearly 1,400,000 have been named but probably many more exist Even so the number of described species is vast and WHAT IS AN INVERTEBRATE it is difficult to get data on how many of these invertebrates are under threat Recent estimates suggest that more than 2,600 are affected but since this is less than twice the number of recognized threatened mammals it is almost certainly an underestimation of the real situation Statistics can be misleading From the figures in the table below in the IUCN Red List 2008 you might think that very few invertebrate species are threatened just 0 5 percent of the total assessed in comparison with 27 percent of mammal species But the number of invertebrate species is huge And of the species studied the percentage assessed as threatened is higher than for mammals Are 35 percent of invertebrates under threat No Scientists could not evaluate all insects as they do mammals so they look at those species that are rarer and more likely to be under threat This display of butterflies and tarantulas is a sad reminder of the continuing trade in animal specimens Numbers of species and percentages of mammals and invertebrates threatened in 2008 Threatened Mammals and Invertebrates No of species in group 5,488 1,400,000 of total in group threatened 27 0 5 Mammals Invertebrates Mammals Invertebrates No of threatened species 1,466 2,639 of total assessed threatened 27 35 7
