The endoderm gives rise to the stomach, intestines, liver, pancreas, and the lining of the digestive tract, as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract. The ectoderm develops into the outer epithelial covering of the body surface and the central nervous system. The mesoderm, the third germ layer forming between the endoderm and ectoderm in triploblasts, gives rise to all muscle tissues including the cardiac tissues and muscles of the intestines , connective tissues such as the skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen.
Differentiation in triploblasts : Triploblasts may be a acoelomates, b eucoelomates, or c pseudocoelomates. Acoelomates have no body cavity. Eucoelomates have a body cavity within the mesoderm, called a coelom, which is lined with mesoderm.
Pseudocoelomates also have a body cavity, but it is sandwiched between the endoderm and mesoderm. Triploblasts that do not develop a coelom are called acoelomates: their mesoderm region is completely filled with tissue. Flatworms in the phylum Platyhelminthes are acoelomates. Eucoelomates or coelomates have a true coelom that arises entirely within the mesoderm germ layer and is lined by an epithelial membrane. This coelomic cavity represents a fluid-filled space that lies between the visceral organs and the body wall.
It houses the digestive system, kidneys, reproductive organs, and heart, and it contains the circulatory system. The epithelial membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some free motion. Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates. The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility and improved animal motility.
The coelom also provides cushioning and shock absorption for the major organ systems, while allowing organs to move freely for optimal development and placement. The pseudocoelomates have a coelom derived partly from mesoderm and partly from endoderm. Although still functional, these are considered false coeloms. The phylum Nematoda roundworms is an example of a pseudocoelomate. Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in their early embryonic development.
These two groups are separated based on which opening of the digestive cavity develops first: mouth protostomes or anus deuterostomes. Early embryonic development in eucoelomates : Eucoelomates can be divided into two groups based on their early embryonic development.
In protostomes, part of the mesoderm separates to form the coelom in a process called schizocoely. In deuterostomes, the mesoderm pinches off to form the coelom in a process called enterocoely.
The coelom of most protostomes is formed through a process called schizocoely, when a solid mass of the mesoderm splits apart and forms the hollow opening of the coelom.
Deuterostomes differ in that their coelom forms through a process called enterocoely, when the mesoderm develops as pouches that are pinched off from the endoderm tissue.
These pouches eventually fuse to form the mesoderm, which then gives rise to the coelom. The majority of species in the phylum Chordata are found in the subphylum Vertebrata, which include many species with which we are familiar.
The vertebrates contain more than 60, described species, divided into major groupings of the lampreys, fishes, amphibians, reptiles, birds, and mammals. Animals in the phylum Chordata share four key features that appear at some stage of their development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail [Figure 3]. In certain groups, some of these traits are present only during embryonic development.
The chordates are named for the notochord , which is a flexible, rod-shaped structure that is found in the embryonic stage of all chordates and in the adult stage of some chordate species. It is located between the digestive tube and the nerve cord, and provides skeletal support through the length of the body. In vertebrates, the notochord is present during embryonic development, at which time it induces the development of the neural tube and serves as a support for the developing embryonic body.
The notochord, however, is not found in the postnatal stage of vertebrates; at this point, it has been replaced by the vertebral column the spine. The dorsal hollow nerve cord is derived from ectoderm that sinks below the surface of the skin and rolls into a hollow tube during development. In chordates, it is located dorsally to the notochord. In contrast, other animal phyla possess solid nerve cords that are located either ventrally or laterally.
The nerve cord found in most chordate embryos develops into the brain and spinal cord, which compose the central nervous system. Pharyngeal slits are openings in the pharynx, the region just posterior to the mouth, that extend to the outside environment.
In organisms that live in aquatic environments, pharyngeal slits allow for the exit of water that enters the mouth during feeding. Some invertebrate chordates use the pharyngeal slits to filter food from the water that enters the mouth. In fishes, the pharyngeal slits are modified into gill supports, and in jawed fishes, jaw supports.
In tetrapods, the slits are further modified into components of the ear and tonsils, since there is no longer any need for gill supports in these air-breathing animals. Birds are considered tetrapods because they evolved from tetrapod ancestors. The post-anal tail is a posterior elongation of the body extending beyond the anus. The tail contains skeletal elements and muscles, which provide a source of locomotion in aquatic species, such as fishes.
In some terrestrial vertebrates, the tail may also function in balance, locomotion, courting, and signaling when danger is near. In many species, the tail is absent or reduced; for example, in apes, including humans, it is present in the embryo, but reduced in size and nonfunctional in adults.
Which of the following statements about common features of chordates is true? In addition to the vertebrates, the phylum Chordata contains two clades of invertebrates: Urochordata tunicates and Cephalochordata lancelets. Members of these groups possess the four distinctive features of chordates at some point during their development. The tunicates [Figure 4 ] are also called sea squirts. The name tunicate derives from the cellulose-like carbohydrate material, called the tunic, which covers the outer body.
Although tunicates are classified as chordates, the adult forms are much modified in body plan and do not have a notochord, a dorsal hollow nerve cord, or a post-anal tail, although they do have pharyngeal slits. The larval form possesses all four structures. Most tunicates are hermaphrodites. After hatching, a tunicate larva swims for a few days until it finds a suitable surface on which it can attach, usually in a dark or shaded location.
Adult tunicates may be either solitary or colonial forms, and some species may reproduce by budding. Most tunicates live a sessile existence on the ocean floor and are suspension feeders. However, chains of thaliacean tunicates called salps Figure can swim actively while feeding, propelling themselves as they move water through the pharyngeal slits. The primary foods of tunicates are plankton and detritus. Suspended material is filtered out of this water by a mucous net produced by the endostyle and is passed into the intestine via the action of cilia.
The anus empties into the excurrent siphon, which expels wastes and water. Tunicates are found in shallow ocean waters around the world.
A cranium is a bony, cartilaginous, or fibrous structure surrounding the brain, jaw, and facial bones Figure. Vertebrates are named for the vertebral column, composed of vertebrae —a series of separate, irregularly shaped bones joined together to form a backbone Figure. Initially, the vertebrae form in segments around the embryonic notochord, but eventually replace it in adults. In most derived vertebrates, the notochord becomes the nucleus pulposus of the intervertebral discs that cushion and support adjacent vertebrae.
Traditional phylogenies place the cephalochordates as a sister clade to the chordates, a view that has been supported by most current molecular analyses. This hypothesis is further supported by the discovery of a fossil in China from the genus Haikouella.
This organism seems to be an intermediate form between cephalochordates and vertebrates. The Haikouella fossils are about million years old and appear similar to modern lancelets. These organisms had a brain and eyes, as do vertebrates, but lack the skull found in craniates. Vertebrates are the largest group of chordates, with more than 62, living species, which are grouped based on anatomical and physiological traits.
More than one classification and naming scheme is used for these animals. Virtually all modern cladists classify birds within Reptilia, which correctly reflects their evolutionary heritage. Thus, we now have the nonavian reptiles and the avian reptiles in our reptilian classification. We consider them separately only for convenience. Further, we will consider hagfishes and lampreys together as jawless fishes, the Agnatha , although emerging classification schemes separate them into chordate jawless fishes the hagfishes and vertebrate jawless fishes the lampreys.
Tetrapods include amphibians, reptiles, birds, and mammals, and technically could also refer to the extinct fishlike groups that gave rise to the tetrapods.
Tetrapods can be further divided into two groups: amphibians and amniotes. Amniotes are animals whose eggs contain four extraembryonic membranes yolk sac, amnion, chorion, and allantois that provide nutrition and a water-retaining environment for their embryos. Amniotes are adapted for terrestrial living, and include mammals, reptiles, and birds. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms.
Most tunicates live on the ocean floor and are suspension feeders. Which of the two invertebrate chordate clades is more closely related to the vertebrates continues to be debated. Vertebrata is named for the vertebral column, which is a feature of almost all members of this clade. The name Craniata organisms with a cranium is considered to be synonymous with Vertebrata. Figure Which of the following statements about common features of chordates is true? Which of the following is not contained in phylum Chordata?
Hagfish, lampreys, sharks, and tuna are all chordates that can also be classified into which group? The characteristic features of the phylum Chordata are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. What is the structural advantage of the notochord in the human embryo? Be sure to compare the notochord with the corresponding structure in adults.
In the adults, the notochord has been replaced by the bony, rigid vertebral column. This loss of flexibility restricts the movement of adult humans, and would make it unlikely that the embryo would fit within the small space it is allotted inside the uterus.
Skip to content Vertebrates. Learning Objectives By the end of this section, you will be able to do the following: Describe the distinguishing characteristics of chordates Identify the derived characters of craniates that sets them apart from other chordates Describe the developmental fate of the notochord in vertebrates.
0コメント