Science, 5th Unit 2 (part 1 - Vertebrates Animals created on @flowvella Science, 5th Unit 2 (part 1 - Vertebrates Animals - Screen 6 on FlowVella - Presentation Software for Mac iPad and iPhone FlowVella. Arthropod, any member of the phylum Arthropoda, the largest phylum in the animal kingdom, which includes such familiar forms as lobsters, crabs, spiders, mites, insects, centipedes, and millipedes. About 84 percent of all known species of animals are members of this phylum.
- Arthropods and Echinoderms created on @flowvella. Sign up for FlowVella. Sign up with Facebook. Already have an account?
- Dec 24, 2019 The text of this page is licensed under the creative commons attribution noncommercial license version 30. Polyphemus from patten 1894 with his original labels note that our current understanding of the xiphosuran nervous system is different from pattens view in several aspects such as the primary olfactory organ or close parallels of the xiphosuran with the vertebrate brain.
Arthropods are a group of animals forming the phylum Euarthropoda. The group includes insects, crustaceans, myriapods, and arachnids. They are all invertebrate animals.
Segmented body, paired jointed appendages, and the presence of an exoskeleton are some of the features characterizing the phylum. Most of these animals are bilaterally symmetrical. They exhibit the greatest diversity among all animal groups. Arthropods account for more than 80% of all extant species on Earth. The species are distributed in a variety of ecosystems ranging from marine and freshwater to land and air.
Some of the main characteristics of arthropods have been mentioned below:
7. Reproduction In Arthropods
Except for a few species, most arthropods have separate sexes. Each sex has paired sex organs that open onto the ventral surface of the trunk via ducts. Sperms in these creatures are usually protected within sealed packets called spermatophores that ensure that the sperms are not diluted by water in aquatic environments and not desiccated in terrestrial environments. In some species, spermatophores are released on the ground while in others they are deposited during a nuptial dance between the species. However, in most species, the males deposit them directly into the female genital opening during copulation. Some species have specialized and unique methods of reproduction. In some species, parthenogenesis can be observed while others give birth to living young instead of eggs.
6. Nervous System In Arthropods
Arthropods have a very basic nervous system consisting of a brain located dorsally and a ganglionated longitudinal nerve cord on the ventral side. Nerves extend laterally into each segment from the nerve cord.
5. Open Circulatory System In Arthropods
Arthropods have an open circulatory system. Their heart is placed dorsally. The arthropods have either an extensive or limited arterial system that delivers blood into hemocoels or tissue spaces. The heart and the arteries are linked via paired openings or ostia where blood movement is controlled by valves. Crustaceans and arachnids have blue blood as it contains the oxygen-carrying pigment hemocyanin. In insects, such pigments are lacking since oxygen is delivered to the tissues directly from the tracheal system. Hemoglobin is, however, present in the blood of a few small crustacean species and insect larvae.
Cached
4. Respiratory System In Arthropods
Terrestrial arthropods possess tracheae and book lungs as respiratory organs. The former is a network of tiny tubes that allow gases to find their way to the interior of the body. The latter are internal pockets lined by chitin with blood-filled plates. Air brought in by the tracheae circulates over these plates allowing the exchange of gases. Aquatic arthropods, on the other hand, have gills for respiration. Although the gills which are outgrowths of the skin are covered by an exoskeleton, it is thin in this area permitting gaseous exchange.
3. Diverse Feeding Habits
Arthropods have diverse feeding habits. There are herbivores, carnivores, parasites, detritus feeds, etc., in this group. Usually, appendages that aid in feeding are present around the mouth. Each species has appendages specialized per the dietary habit of the species. For example, fiddler crabs have small claws that allow them to scoop up surface sand and place it within their mouth where tiny hairs are used to sift the sand to keep the organic parts and expel the mineral material. On the other hand, aphids have appendages that are used to pierce vegetation to suck out plant juices.
2. Locomotion
Unlike most other invertebrates, arthropods lack locomotory cilia at all stages of life. They move with the co-ordinated action of their exoskeleton, appendages, and muscles. Their exoskeleton is segmented to allow for movement. The appendages of these animals are specially adapted to allow different types of locomotion like walking, swimming, burrowing, running, etc. Some insects also possess wings that help them fly.
1. Exoskeleton
Arthropods possess a hard, nonliving, exoskeleton that not only provides protection and support of the creature but also helps in locomotion. The exoskeleton is made of two layers, an inner thick chitin-protein layer (procuticle) and an outer protein layer (epicuticle). To allow the arthropod to grow, the exoskeleton is periodically shed in a process called molting or ecdysis, and new exoskeleton is generated.
What has more species than any other animal phylum?
Arthropods are not only the largest phylum of invertebrates. They are by far the largest phylum of the animal kingdom. Roughly 80 percent of all animal species living on Earth today are arthropods. Obviously, arthropods have been extremely successful. What accounts for their success?
Arthropods
There are more than a million known species of arthropods. There may actually be ten times that many. Arthropods include insects, spiders, lobsters, and centipedes. The arthropods pictured in Figurebelow give just a hint of the phylum’s diversity.
Arthropod Diversity. Dust mites are among the smallest of arthropods. Japanese spider crabs are the largest. Besides size, what other differences among arthropods do you see in these photos?
Structure and Function of Arthropods
Arthropods range in length from about 1 millimeter to 4 meters (about 13 feet). They have a segmented body with a hard exoskeleton. They also have jointed appendages. The body segments are the head, thorax, and abdomen (see Figurebelow). In some arthropods, the head and thorax are joined together as a cephalothorax.
Arthropod Body Plan. Notice the three body segments of each organism.
The arthropod exoskeleton consists of several layers of cuticle. The exoskeleton prevents water loss and gives support and protection. It also acts as a counterforce for the contraction of muscles. The exoskeleton doesn’t grow as the animal grows. Therefore, it must be shed and replaced with a new one periodically through life. This is called molting.
The jointed appendages of arthropods may be used as legs for walking. Being jointed makes them more flexible. Try walking or climbing stairs without bending your knees, and you’ll see why joints are helpful. In most arthropods, the appendages on the head have been modified for other functions. Figurebelow shows some of head appendages found in arthropods. Sensory organs such as eyes are also found on the head.
Arthropod Head. Arthropods have evolved a variety of specialized appendages and other structures on their head.
Some arthropods have special excretory structures. They are called coxal glands and Malpighian tubules. Coxal glands collect and concentrate liquid waste from blood. They excrete the waste from the body through a pore. Malphigian tubules carry waste from the digestive tract to the anus. The waste is excreted through the anus.
Like mollusks and annelids, aquatic arthropods may have gills to exchange gases with thewater (discussed below). Terrestrial arthropods, on the other hand, have special respiratory structures to exchange gases with the air. These are described in Figurebelow.
How Terrestrial Arthropods Breathe Air. Terrestrial arthropods have respiratory structures that let them breathe air.
See Full List On Earthlyuniverse.com
Underwater Spiders
In the ponds of northern Europe lives a tiny brown spider that spends its entire life underwater. But just like land spiders, it needs oxygen to breathe. So, how does this spider breath? Does it use book lungs? No. In fact, aquatic spiders, known as 'diving bell spiders,' have gills. Every so often, the spider leaves its underwater web to visit the surface and bring back a bubble of air that sticks to its hairy abdomen. It deposits the bubble into a little silk air tank. This 'diving bell' is a gill that sucks oxygen from the water, allowing the spider to stay underwater for up to 24 hours.
Arthropod Reproduction
Arthropods have a life cycle with sexual reproduction. Most species go through larval stages after hatching. The larvae are very different from the adults. They change into the adult form in a process called metamorphosis. This may take place within a cocoon. A familiar example of metamorphosis is the transformation of a caterpillar (larva) into a butterfly (adult). Other arthropod species, in contrast, hatch young that look like small adults. These species lack both larval stages and metamorphosis.
Evolution of Arthropods
11.10: Arthropods - Biology LibreTexts
The oldest known arthropods are trilobites. A fossil trilobite is shown in Figurebelow. Trilobites were marine arthropods. They had many segments with paired appendages for walking. As arthropods continued to evolve, segments fused. Eventually, arthropods with three major segments evolved. Appendages were also lost or modified during the course of arthropod evolution.
Trilobite Fossil. This trilobite fossil represents the earliest arthropods. Trilobites first appeared more than 500 million years ago. They lived for at least 200 million years before going extinct. They left behind large numbers of fossils.
Arthropods were the first animals to live on land. The earliest terrestrial arthropods were probably millipedes. They moved to land about 430 million years ago. Early land arthropods evolved adaptations such as book lungs or trachea to breathe air. The exoskeleton was another important adaptation. It prevents an animal from drying out. It also provides support in the absence of buoyant water.
Classification of Arthropods
Living arthropods are divided into four subphyla. They are described in Tablebelow. The Hexapoda subphylum includes mainly insects. There are so many insects and they are so important that they are described in greater detail below.
| Subphylum (includes) | Description | Example |
|---|---|---|
| Myriapoda (centipedes, millipedes) | terrestrial; herbivores or predators; 10–400 walking legs; poison claws for hunting | centipede |
| Chelicerata (spiders, scorpions, mites, ticks, horseshoe crabs, sea spiders) | mainly terrestrial; predators or parasites; 8 walking legs; appendages called chelicerae for grasping prey; poison fangs for killing prey; no mandibles, maxillae, antennae; two body segments | spider |
| Crustacea (lobsters, crabs, shrimp, barnacles, krill) | mainly aquatic, predators, scavengers, or filter feeders; two pairs of antennae and claws for hunting; unique larval stage (called “nauplius”) with head appendages for swimming | lobster |
| Hexapoda (ants, flies, grasshoppers, beetles, butterflies, moths, bees, springtails) | mainly terrestrial or aerial; herbivores, predators, parasites, scavengers, or decomposers; 6 walking legs; many modified appendages, such as wings for flying | beetle |
Summary
- Arthropods are the largest phylum in the animal kingdom.
- Most arthropods are insects. The phylum also includes spiders, centipedes, and crustaceans.
- The arthropod body consists of three segments with a hard exoskeleton and jointed appendages.
- Terrestrial arthropods have adaptations for life on land, such as trachea or book lungs for breathing air.
- The earliest arthropods were trilobites. The earliest land arthropods were millipedes.
Review
- Identify the distinguishing trait of arthropods.
- What is molting? Why does it occur?
- What are the Malphigian tubules?
- Describe two structures that allow arthropods to breathe air.
- Assume you see a “bug” crawling over the ground. It has two body segments and lacks antennae. Which arthropod subphylum does the “bug” belong to? Explain your answer.