FACTS AT YOUR FINGERTIPS ENDANGERED ANIMALS INVERTEBRATES BROWN BEAR BOOKS
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
