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Phylum Chordata - Study Here
1. What are the two main subdivisions of the phylum Chordata? The phylum Chordata is divided into protochordates (urochordates and cephalochordates) and vertebrates (cyclostomes, fishes, amphibians, reptiles, birds and mammals). Image Diversity: urochordates cephalochordates 2. What are the three structures shared by every chordate that characterize the group? All beings of the phylum Chordata have branchial clefts in the pharynx (in some species present only in the embryo), notochord (substituted by the spinal column in vertebrates) and dorsal neural tube. Image Diversity: branchial clefts notochord dorsal neural tube 3. What is the destination of the branchial clefts in humans? In humans the branchial clefts located in the anterior region of the pharynx (also known as pharyngeal clefts) are present only in the embryonic stage and disappear later. 4. What is the destination of the notochord in vertebrates and in protochordates? In vertebrates the notochord disappears and gives birth to the spinal column (vertebral column). In protochordates the notochord remains for the rest of the life. 5. Is the tubular-dorsal nervous system of chordates associated to radial or lateral symmetry? How does that explain the complexity level of the nervous system reached by the vertebrate evolutionary branch? The tubular and dorsal nervous system of chordates is related to the cephalization in these animals and to bilateral symmetry. The presence of neural integrating centers with concentration of neurons in the brains and the spinal cord (central nervous system, CNS) allowed the increase of the interaction complexity between these animals and the environment. Receptor (afferent conduction) and efferent (motor, regulatory and behavioral reactions) functions come out more sophisticated in chordates due to the presence of more well-developed neural networks. These features have been preserved by evolution as they provide adaptive advantage to their owners. 6. How can the tubular-dorsal nervous system in chordates be compared to the nervous pattern present in invertebrates? In chordates the nervous system is dorsal and highly cephalized while in most invertebrates the nervous system is ganglial and ventral. 7. What is the group of the phylum Chordata that first colonized the terrestrial environment? From which habitat did they come? Amphibians, partially aquatic partially terrestrial animals, were the first chordates that colonized the dry land. They came from the aquatic habitat and were originated from fishes. (Nevertheless the first completely terrestrial chordates were the reptiles). 8. How do chordates reproduce? Reproduction in beings of the phylum Chordata is sexual, with the exception of urochordates that can also reproduce asexually. In some classes (cyclostomes, osteichthyes fishes and amphibians) there is larval stage. With rare exceptions, fishes, amphibians, reptiles and monotreme mammals are oviparous, egg-laying (embryos develop within eggs and outside the mother’s body) and marsupial and placental mammals are viviparous (embryos develop inside the mother’s body, feeding from her). 9. Into which subphyla are the protochordates divided? What are some representatives of each protochordate subphylum? Urochordates (or tunicates) and cephalochordates are the two subphyla into which protochordates are divided. Ascidians, sessile animals similar to sponges, are examples of tunicates. The amphioxus, well-studied in Embryology, is an example of cephalochordate. 10. What are the six criteria used to build a complete evolutionary branch of vertebrates? Dichotomy in each of the six following criteria builds the vertebrate evolutionary branch: absence of mandibles separates cyclostomes from others; absence of limbs separates fishes from the remaining; absence of osseous skeleton separates chondrichthian (cartilaginous) fishes from osteichthyes; absence of impermeable skin separates amphibians from the terrestrial vertebrates; absence of warm blood (homeothermic body) separates reptiles from birds and mammals; absence of mammary glands and hair separates birds from mammals. Image Diversity: cyclostomes 11. Evolutionarily protochordates may be intermediate animals between invertebrates and vertebrates. Imagine a scientist is testing the hypothesis that vertebrates evolved from echinoderms. Having as research object a newly found protochordate species, what is an example of a discovery that would weaken the hypothesis? What is an example of a discovery that would strengthen it? Hypothesis to be tested: Vertebrates evolved from echinoderms. Testing material: A newly found protochordate species (i.e., a vertebrate forerunner). v Example of weakening observation (that puts the new protochordate species evolutionarily far from echinoderms): “The new species is protostome”, as opposed to echinoderms, that are deuterostomes. This observation brings the suspicion that deuterostomy in vertebrates is independent in origin from echinoderms. Example of strengthening observation (that nears the new protochordate species to echinoderms): “The new species has secondary radial symmetry”, similar to echinoderms. This observation makes stronger the hypothesis that echinoderms and vertebrates are relatives. (This item shows how science works, beginning with the placement of hypothesis and further observational testing.) 12. Chordate identity card. How are they characterized according to examples of representing beings, basic morphology, type of symmetry, germ layers and coelom, digestive system, respiratory system, circulatory system, excretory system, nervous system and types of reproduction? Examples of representing beings: protochordates (ascidians, amphioxus), vertebrates. Basic morphology: branchial clefts, notochord, neural tube. Type of symmetry: bilateral. Germ layers and coelom: triploblastics, coelomates. Digestive system: complete, deuterostomes. Respiratory system: branchial (in aquatic), cutaneous (in adult amphibians), pulmonary (others). Circulatory system: open in protochordates, closed in vertebrates. Excretory system: diffusion and flame cells in protochordates, kidneys in vertebrates. Nervous system: neural tube in embryos, cerebral vesicle and single ganglion in protochordates, brain within the cranium and spinal cord within the spinal column in vertebrates. Types of reproduction: sexual, with or without larval stage. |