Opisthokonta free essay sample

Opisthokonta: Metazoa II: Eumetazoa: Bilateria: Protostomia: Ecdysozoa Objectives: †¢ Know the key features of the Phylum Nematoda. Know how they relate to other metazoan phyla. †¢ List the key characteristics of the Phylum Arthropoda. Identify examples. List the four subphyla and their key distinguishing features. Identify examples of the subphyla Chelicerata, Crustacea, Myriapoda, and Hexapoda. †¢ Be able to identify the different structures on these organisms and understand their function. †¢ Understand differences between terrestrial versus aquatic arthropods (e. g. grasshoppers versus crawfish) and how that relates to environment in which they live. †¢ Define and be able to apply the terms in bold found throughout this exercise. Introduction: Our survey of metazoan diversity continues with the clade Bilateria, which consists of animals which have bilaterally symmetrical body plans, unlike the radially symmetrical Phylum Cnidaria and the largely asymmetrical Phylum Porifera. Additionally, members of this clade are triploblastic, meaning that they have three embryonic tissue layers from which structures and organs develop, in contrast to the diploblastic cnidarians. The clade Bilateria can itself be divided into two additional clades based on differences in developmental pathways: protostomes and deuterostomes. At an early stage of embryonic development, around the 128–cell stage, eumetazoans form a hollow sphere of cells referred to as a blastula. In the next developmental step, cell divisions on one side of the sphere result in inward growth toward the hollow interior of the sphere. This developmental step is known as gastrulation, the embryonic stage is known as the gastrula, and the initial opening on the outside of the sphere of cells where the inward growth is known as the blastopore.In most bilaterian animals, the inward growth continues and emerges as a second opening on the opposite side of the gastrula. In protostomes, the blastopore will develop into the mouth of the juvenile and adult stages of the organism. The name is derived from the word roots which mean ? first mouth? , i. e. the mouth is formed first. The second opening formed becomes the anus. The situation is reversed in deuterostomes (? second mouth? ). Evidence supports division of the clade Protostomia into two clades: Ecdysozoa and Lophotrochozoa.The clade Ecdysozoa comprises one of the most species-rich groups within the animal kingdom and includes eight phyla. This includes the Phylum Arthropoda (including insects, arachnids, and crustaceans) and the Phylum Nematoda (commonly called nematodes or roundworms). Many of the animals you previously examined, such as sponges and corals, and many that you will examine in the future, such as chordates, build their bodies’ skeletal support from minerals like calcium carbonate or silica. A unique feature of ecdysozoans is that they instead build a cuticle, a non-living, outer layer of organic material that functions as its skeleton.The cuticle serves to both support and protect the animal. It can be built thinner and lighter than in other animals, and does not require a source of minerals for its construction. The fact that a cuticle can be very thin means it does not require joints like a mineral skeleton in order to allow flexibility. The name Ecdysozoa refers to the fact that members of this group regularly shed their cuticle, a process called ecdysis that is controlled hormonally by a class of steroids appropriately called ecdysteroids.Although their name is based upon a morphological trait, there is much molecular data to support the g rouping of ecdysozoans into a monophyletic clade, such as the presence of similar ecdysteroids in nematodes and arthropods. If you have ever seen an insect crawl out of its old skin, or a butterfly leaving its chrysalis, then you have witnessed ecdysis. This ability to shed the outer skeleton has opened up developmental options for ecdysozoans that other animals with skeletons do not have available to them.This is in part because of the limitations a mineral skeleton imposes on an animal; growth can only occur by adding more mineral to the existing skeleton, which limits the animals form as it grows. While many ecdysozoans also maintain their basic form throughout their life, molting removes this limitation. Some ecdysozoans have taken advantage of this, especially the insects. Most groups of insects undergo partial or complete metamorphosis before reaching the adult stage (Figs. 1 and 2), and the larva may look very different from the final adult and even live in a completely different environment. Figure 1 Incomplete metamorphosis Figure 2 Complete metamorphosis Also, unlike basal animals (Porifera and Cnidaria), and many lophotrochozoans and deuterostomes who reproduce by simply releasing mass quantities of egg and sperm cells into the water in the hopes that fertilization will occur, many species of ecdysozoans have separate sexes who come together for copulation. The sperm is either delivered to the females body, as in internal fertilization, or is deposited directly onto the eggs as they are released. This trait greatly increases the chances of successful fertilization, and has evolved independently in the vertebrates.Early embryonic stages of Bilateria Observe the slides labeled ? starfish development w. m.?. Using the photomicrographs of starfish developmental stages provided, find, draw and label the blastula, gastrula, blastopore, blastocoel and archenteron. What structure will the blastopore develop into in the adult animal? What about the blastocoel? the archenteron? starfish developmental stages magnification. Phylum Arthropoda Arthropods, literally meaning ? jointed feet? , represent the most successful and abundant organisms in the animal kingdom.About three quarters of all described living species are arthropods and estimates of their numerical abundance are as high as 1018 (a billion billion! ). They are important components of both terrestrial and aquatic habitats, and contribute ecologically and economically to our everyday lives by playing roles such as pollinators, biological control agents, delicious dinners and commodity producers (e. g. silkworm moth caterpillars). However, some species also damage crops and wooden structures, and carry disease. Arthropods live in all major habitats and their incredible success can be attributed to a number of characteristics.The phylogeny of arthropods is the subject of much debate in the scientific community and continues to change as new molecular data becomes available (see www. tolweb. org for recent advances). As ecdysozoans, arthropods possess a jointed cuticle made of various salts and chitin, a polysaccharide with nitrogen cross-links. Their cuticle is often referred to as an exoskeleton and can vary in composition along the body, allowing for some regions of the exoskeleton to be rigid and brittle and other regions to be more soft and flexible. This variation in strength and flexibility allows arthropods to inhabit many different environments and ill a wide range of ecological niches. In addition to providing protection and structural support, the exoskeleton reduces moisture loss and provides a surface for muscle attachment. When an arthropod grows, and undergoes ecdysis, it will distend its body, either by taking in extra air or water, thereby rupturing the cuticle. After an arthropod has molted, the new cuticle is soft and requires a few hours or days to harden. During this time, arthropods are particularly vulnerable to predators. Arthropods also have paired, jointed appendages.Appendages are covered by the rigid exoskeleton but usually have flexible joints. In many species, these appendages are highly modified to perform specific functions related to locomotion, feeding, reproduction, and defense. Again, these variations allow arthropods to succeed in many different environments. Arthropods exhibit metamerism. The body plan has repeated units, or metameres. The repetition and redundancy of body structures observed in metamerism allows structures and regions of the body to become differentiated for the purpose of specialization for various functions.Specialization of segments or fused groups of segments also allows for an efficient division of labor among body parts; i. e. locomotion, feeding, and defense. This evolutionary flexibility contributed to the diversification of arthropods. Most arthropods have two or three distinct body regions, or tagmata. The head of the arthropod usually bears 3-5 pairs of appendages that are involved in feeding and sensory function. Typically, the first pair of appendages is a pair of antennae, which are sensory structures. The next pair are the mandibles (used for biting or chewing), followed by the maxillae (which also aid in feeding).In arachnids, horseshoe crabs, and pycnogonids, there are no antennae or mandibles. The first pair of appendages is comprised of two chelicerae, or pincers, and the next pair is composed of two pedipalps, which are used for sensory function, feeding and mating. The thoracic, or middle, body segments usually bear appendages that are used in walking or swimming. The abdominal, or posterior, segments may or may not have appendages, and in the case of some aquatic crustaceans, may end in a telson, or tail-like structure. Tagmatization is not seen in millipedes and centipedes.Arthropods have an open circulatory system, meaning their blood is not kept separate from their interstitial fluid, but is instead combined to form a fluid known as hemolymph that surrounds the organs and permeates the tissues. Respiratory exchange occurs through gills in most aquatic arthropods and tracheal tubes (Fig. 3) or book lungs in terrestrial arthropods. Many arthropods also have a high degree of cephalization with complex sensory organs, a welldeveloped brain, and compound eyes, the last of which are unique to arthropods. They are composed of numerous closely packed units, called ommatidia, each of which contains a bundle of light-sensitive cells. Figure 3 Insects breathe through a tracheal tube system. Tracheal tubes, such as the one pictured above, carry oxygen from spiracles, small openings on the surface of the insects body, to branching tracheoles, which deliver oxygen to the tissues. Subphylum Trilobita Trilobites are ancient arthropods that dominated the muddy bottoms of the shallow seas during the Paleozoic, over 500 million years ago.Although they have been extinct for over 250 million years, they display many of the typical features of living arthropods including a chitinous exoskeleton, fused segments, and branched appendages. Trilobites were dorsoventrally flattened and divided into three longitudinal sections (hence the name of the group): two lateral lobes and a ventral lobe. In addition they have three anterio-posterior regions: the head, the thorax, and abdomen. Each body segment has two biramous (two-branched) appendages. One branch was adapted for walking/crawling, while the other had gills.