Major Types of Arthropods Help
Introduction to Major Types of Arthropods
There are several major types or categories of arthropods. The first question a person might ask, however, is: “The arthropods are all invertebrates. So, why weren’t they included within Chapter 10, which described the invertebrates?” [ Study suggestion: Take a careful look at Figure 11.2. What particular feature really sticks out at you? How do you think this feature explains why arthropods are placed in a separate chapter of their own?]
Using a traditional approach to taxonomy, there is a single phylum Arthropoda (ar- THRAH -pah-da), subdivided into five smaller classes. These are the Crustaceans (krus- TAY -shuns), Arachnids (ah- RACK -nids), Chilopods ( KY -luh-pods), Diplopods ( DIP -luh-pods), and Insects. In Figure 11.2, the relative size of each oval shape representing these arthropod classes visually pictures the number of known species belonging to each one.
Ancient Trilobites And The Modern Crustaceans
Let us begin our examination of the major arthropod classes with the crustaceans – invertebrates whose bodies are surrounded by hard “shells” (crustace). In the modern world, the crustaceans include crabs, lobsters, crayfish, and shrimp. Besides their hard shells (rigid exoskeletons), these crustaceans have jointed bodies and legs, as well as gills that allow them to exchange their hemolymph gases with the external gases in the surrounding water.
There are now about 40,000 known species of crustaceans, most of them living within either freshwater or saltwater environments. Yet, biologists have repeatedly observed an early ancestor (long extinct) of the modern crustaceans in the Fossil Record. This extinct group of arthropods is the trilobites ( TRY -loh-bites). The trilobites consisted of about 4,000 species, and they were very abundant in the sea during the Paleozoic Era (Chapter 3). The trilobites had bodies comprised of “three” (tri-) vertical “lobes” (lob) subdivided into many horizontal segments (Figure 11.3). Sprouting from either side of the body segments were numerous jointed legs. But unlike the modern lobster and other crustacea, these ancient marine invertebrates had appendages running along either side of their body that showed very little difference or specialization. For instance, they lacked the protective pincers of present-day crabs and lobsters.
The trilobites rather suddenly became extinct about 250 million years ago, near the end of the Paleozoic Era (shortly before the dinosaurs appeared during the Age of Reptiles). Because they never reappear at later stages in the Fossil Record, the limited time period of trilobite existence strongly supports the concept that once a group of animals becomes extinct, they generally stay extinct! There is a definite Biological Order or recognizable sequence of body forms and patterns that seem to precede or follow one another during the long time-scale of evolution.
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