2. What three structures are present among all chordates and are characteristic of this phylum?
All organisms of the phylum Chordata have branchial clefts in the pharynx (in some species present only in the embryo), a notochord (substituted by the spine in vertebrates) and a dorsal neural tube.
Chordates Review - Image Diversity: branchial clefts notochord dorsal neural tube
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3. Where are branchial clefts found in humans?
In humans, the branchial clefts located in the anterior region of the pharynx (also known as pharyngeal clefts) are present only during the embryonic stage and disappear later.
4. What happens to the notochord in vertebrates and protochordates?
In vertebrates, the notochord disappears and to produce the spine (vertebral column). In protochordates, the notochord remains during their whole life.
The Dorsal Neural Tube
5. Is the tubular-dorsal nervous system of chordates associated with radial or lateral symmetry? How does that explain the level of nervous system complexity attained 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 a concentration of neurons in the brain and the spinal cord (central nervous system, CNS) permitted the increase in the complexity of the interaction between these animals and their environment. Receptor (afferent conduction) and efferent (motor, regulatory and behavioral reactions) functions are more sophisticated in chordates due to the presence of more better-developed neural networks. These features have been preserved by evolution, as they provide an adaptive advantage to the species in which they are present.
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. In most invertebrates, the nervous system is ganglial and ventral.
Chordate Evolution and Reproduction
7. What group of the phylum Chordata first colonized the terrestrial environment? What was its original habitat?
Amphibians, partially aquatic and partially terrestrial animals, were the first chordates to live on dry land. They originated from an aquatic habitat and are a descendant of fish. (Nevertheless, the first completely terrestrial chordates were reptiles).
8. How do chordates reproduce?
Reproduction in organisms of the phylum Chordata is sexual, with the exception of urochordates, which can also reproduce asexually. Some classes (cyclostomes, osteichthyes fish and amphibians) have a larval stage. With rare exceptions, fish, amphibians, reptiles and monotreme mammals are oviparous; egg-laying (embryos develop within eggs and outside the mother’s body), marsupial and placental mammals are viviparous (embryos develop inside the mother’s body, feeding from her).
9. Into which subphyla are protochordates divided? What are some representative species 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 a cephalochordate.
10. What are the six criteria used to determine the evolutionary branch of vertebrates?
The differences with respect to each of the six following criteria determine the evolutionary branch of vertebrates: the absence of mandibles separates cyclostomes from the others; the absence of limbs separates fish from the remainder; the absence of an osseous skeleton separates chondrichthyan (cartilaginous) fish from osteichthyes; the absence of waterproof skin separates amphibians from terrestrial vertebrates; the absence of warm blood (a homeothermic body) separates reptiles from birds and mammals; and the absence of mammary glands and hair separates birds from mammals.
11. Evolutionarily, protochordates may be intermediate animals between invertebrates and vertebrates. Imagine that a scientist is testing the hypothesis that vertebrates evolved from echinoderms. If he/she is studying a newly found protochordate species, what is an example of a discovery that would weaken this 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 (more specifically, a vertebrate precursor).
Example that would weaken the hypothesis (making the new protochordate species evolutionarily distant from echinoderms): discovering that the new species is protostome, as opposed to echinoderms, which are deuterostomes. This observation raises the possibility that deuterostomy in vertebrates is independent in origin from that of echinoderms.
Example that would strengthen the hypothesis (making the new protochordate species evolutionarily closer to echinoderms): discovering that the new species has secondary radial symmetry, which is similar to echinoderms. This observation strengthens the hypothesis that echinoderms and vertebrates are relatives.
(This section shows how science works, beginning with the establishment of a hypothesis and continuing with further observational testing.)
A Summary of Chordates
12. The main features of chordates. How can they be described according to examples of representative species, 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 species: protochordates (ascidians, amphioxus), vertebrates. Basic morphology: branchial clefts, a notochord, a neural tube. Type of symmetry: bilateral. Germ layers and coelom: triploblastic, coelomates. Digestive system: complete, deuterostomes. Respiratory system: branchial (in aquatic chordates), 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 spine in vertebrates. Types of reproduction: sexual, with or without a larval stage.