Slide 1 Our last group of protostome animals are the arthropods.
Slide 2 This is diverse group of organisms with over 1 million described species and potentially 10 million more yet to be described. Two out of three known animal species are arthropods and most of the unknown species will be found in tropical rainforests.
Slide 3 The diversity and success of the arthropods are attributed to three main features of the arthropods. A segmented body (notice that the segmentation of the arthropods is much more advanced than the segmentation seen in the annelids). Jointed appendages allow for greater mobility and more advanced behaviors and a hard exoskeleton made of chitin that provides physical support, protection from trauma, and prevention of dessication.
Slide 4 The body segments in the arthropods occur in three distinct segments - a head, thorax, and abdomen. This segmentation allows for greater body and metabolic specialization. Different parts of the body can move and function independently of each other and this provides great advantages to the arthropods and other organisms with advanced body segmentation. In some arthropods, such as this spider, the head and thorax are fused together into a structure known as a cephalothorax.
Slide 5 This diagram of a lobster clearly shows the segments of this arthropod. Lobsters have a cephalothorax with a fused head and thorax.
Slide 6 Jointed appendages are another feature of the arthropods that have contributed to their success. These jointed appendages have contributed to greater mobility (imagine trying to run without knees). The appendages have been modified in different species for sensing the environment, feeding, and locomotion. These appendages can also be highly modified for defense, mate attraction, and predation. As an example - we can compare the front appendages of the striped mantis shrimp and the peacock mantis shrimp. The striped mantis shrimp feeds on fish and use the speark like projections on its appendage to spear fish as they swim by. The peacock mantis shrimp, on the other hand, feeds on animals inside shells. The club-like appendage of the peacock mantis can generate incredible crushing force - measured as high as 340 pounds of force.
Slide 7 The hard chitinous cuticle of the arthropods protects the animals from physical injury, provides physical support and a structure for muscle attachment, as well as preventing desiccation. Because the skeleton is on the outside - as the animal grows it must shed its cuticle during an event known as molting. This is a very sensitive time in the life cycle because it takes some time for a new cuticle to replace the one lost through molting. Softshell crab season takes advantage of this molt.
Slide 8 There are four subphyla of arthropods. The first are the trilobites which are all extinct. This fossil shows the distinct segmentation of this animal but the segments are not specialized the way they are in most modern day arthropods.
Slide 9 The chelicerates is comprised of spiders, scorpions, ticks, and mites. They are characterized by having a cephalothorax and an abdomen with 4 pairs of walking legs, one pair of chelicerae, and one pair of pedipalps This is a mainly terrestrial group with the exception of the horseshoe crabs.
Slide 10 Horseshoe crabs are commonly referred to as living fossils - they evolved 450 million years ago and have changed little in that time. You can see in this image that modern day horseshoe crabs look pretty much the same as the fossilized horeshoe crab from hundreds of millions of years ago.
Slide 11 The crustaceans include crabs, lobsters, crayfish, shrimp, pill bugs, and wood lice. This is a mainly aquatic group - with the exception of the pill bugs and wood lice. Some species are semi-aquatic like hermit crabs and fiddler crabs which live on the shore near the water. These organisms have jaw-like mandibles, compound eyes, two pair of antennae and branched appendages.
Slide 12 The uniramians are the largest group of arthropods and include the insects, centipedes, and millipedes. They have jaw-like mandibles and compound eyes like the crustaceans but they have only one pair of antennae and unbranched appendages. This is a mainly terrestrial group - in general, the uniramians are most abundant on land and the crustaceans are most abundant in the water.
Slide 13 The insects are the most species rich group of organisms on earth. It is estimated that there are 10 million species of insects on earth today. They are a mainly terrestrial group but there are a few freshwater species. This group includes the only invertebrate organisms that have the ability to fly. Within the insects - the beetles are the most abundant. In fact, British geneticist J.B.S. Haldane is credited with saying nature has shown an "inordinate fondness for beetles".