Describe the Classification of Arthropoda?
Most arthropods also go through a growth process called metamorphosis, whereby they change their bodies into an entirely different form. The most familiar example is probably the change that caterpillars go through when their larval stage morphs into a pupa stage, and from there into an adult stage known as a butterfly.
Many joint-footed appendages of arthropods represent evolutionary developments that have helped them to become successful. Examples of highly specialized functions performed by modified jointed appendages include sensory antennae, claws, mouth parts for sucking, biting and chewing, fangs, walking legs, and flippers.
The heads of arthropods are well developed, with a concentration of the sensory apparatus at the anterior end of the body. The head bears the eyes, organs that sense smell and touch, as well as specialized mouthparts. There are myriad ways in which arthropods feed, especially when one considers there are over one million different species! Suffice it to say, there are species that are herbivorous, carnivorous, omnivorous, ectoparasitic, and endoparasitic; there are also detritovores (those that eat fragments of organic material), and filter feeders. Each species might have specialized feeding apparatus especially adapted to their primary food source.
Circulation in the arthropods is an open system. A heart pumps blood through vessels into cavities called sinuses, where the blood can bathe the tissues. Returning blood flows through vessels that have one-way valves.
Gas exchange takes place in several different ways among the arthropods. Most arthropods that live on land have branching air tubes called tracheae with openings to the exterior called spiracles. These simple air ducts provide a pathway for air to reach and diffuse into tissues deep within the organism. Spiders and other members have book lungs that function in respiration. Book lungs have layers of gill tissue that structurally resemble a book's pages. Aquatic species such as crabs and shrimp have feather-like gills that lie just below the exoskeleton and function in respiration.
Arthropods excrete metabolic wastes in several different ways. Spiders and insects have special pockets of the gut that remove wastes from the circulating blood. These organs of excretion are called Malpighian tubules. The excretory products are combined with the rest of the undigested material in the digestive tract that passes out through the anus. Crustaceans are able to pass excretory wastes out through thin areas in their exoskeleton cuticle. A green gland in crustaceans regulates salt and water balance.
Nervous systems in arthropods are well developed, as all members have a brain. Some arthropods have complex sense organs. The compound eye represents an important evolutionary development that has contributed to this group's success. A compound eye is made of thousands of individual, independently functioning eye units called ommatidia. Each ommatidium produces its own separate image, and sends it to the brain, which receives and combines all of the images into a composite. This composite image, or mosaic, is thought to detect movement more effectively than the images formed by human eyes, thereby providing the small animal the means of avoiding predatators. Other arthropods have simple eyes that are called ocelli. Ocelli can occur by themselves in some species, or along with compound eyes in others.