Discuss the features that make the vertebrate chordates unique from the invertebrate chordates.

Transcription

1 APS214 (Vertebrates) Question 4 Discuss the features that make the vertebrate chordates unique from the invertebrate chordates. The phylem [phylum!] chordate consists of three subphylems two of which are invertebrate subphylems, and one is a vertebrate subphylem. The invertebrate subphylems are urochordata and the cephalochordate, the other subphylem is vertebrata. Vertebrates have five distinctive features, which occurs sometime during their lifecycle. The notochord lies ventrally from the head almost down to the posterior end of the animal. It still persists in adult fishes. [?agnathans] The dorsal hollow nerve cord forms into the brain and the spinal cord. Pharyngeal slits only form gill slits on the body surface of fishes and of buccal amphibians. The post anal tail is a characteristic feature of vertebrates. The endostyle forms from cells that lay on the floor of the pharynx. It is homologous to the thyroid gland, and controls the concentration of iodine in the body. Vertebrates are though to have evolved from ascidian larvae, a tunicate belonging to the subphylem evolved form the ascidian larvae. The notochord encased the tail, the ascidian mode of locomotion. The dorsal hollow nerve cord controlled the muscle activity. The endostyle cells on the floor of the pharynx produced mucous to trap food particles, and the pharyngeal slits, which the water left through, may have caught any remaining food particles or may even have been involved in gas exchange. Vertebrates differ from invertebrates as vertebrates posssess a vertebrae, although not all vertebrates do, for example the hagfish. Bone is not necessarily a characteristic of vertebrates, as the chondreichthyes have a cartilaginous skeleton. [but have bone elsewhere!]also vertebrates possess a cranium, which separates them from invertebrates. Embryological features developed in vertebrates which separates them from invertebrates. Vertebrates developed a neural crest, sensory placcodes and muscle fibres. These have been very important in the success of vertebrates. Other features which make vertebrates unique from invertebrates are their nervous system and their immune system, when vertebrates evolved from ascidian larvae, they retained the larval nervous system but also took the adult nervous system, creating a dual nervous system, somatic and visceral. The vertebrate immune system is unique [so not unique!] as it has allorecognition, it rejects anything non-self. This feature can be found in invertebrates but only in colonial organisms, for example tunicates when colonial. Although vertebrate chordates evolved from invertebrate chordates, and they may have many features in common, there are also many features that make vertebrates unique from invertebrates. All vertebrates possess a notochord, a dorsal hollow nerve cord, pharyngeal slits, a post-anal tail and an endostyle at some stage in their life cycle. Vertebrates have a dual nervous system, a unique immune system, and embryological

2 features which separate them from invertebrates. [contains some features found in invertebrate chordates] This essay includes sufficient relevant material to demonstrate the point of the question and discusses most of the important relevant material. However, it contains some irrelevant material (it doesn t always address features unique to vertebrates) and some incorrect material (some confusion about bone). Poor introductory and concluding paragraphs. Limited development of ideas and use of examples to illustrate points. Adequately written (but take care with your spelling). Mark = 55

3 APS214 (Vertebrates) Question 4 Discuss the features that make the vertebrate chordates unique from the invertebrate chordates. The closest relatives of the chordates in the phylogenetic tree of the animal kingdom are the echinoderms and the hemichordates. However the chordates are distinguished from these groups by the presence of a true notochord, the unique feature of the dorsal hollow nerve cord, pharyngeal pouches or silts, a post-anal tail and also an endostyle or a similar homologous feature, the thyroid gland. There are three sub-classes of chordate two of which are invertebrate (protochordates) and one of which is vertebrate. The aim of this essay is to discuss the features of the vertebrate chordates that distinguish them from the protochordates and to mention the exceptions to the general features. The two protochordate classes are the Cephalochordata (Lancelets) and the Urochordata and it is from the Urochordata that the vertebrate chordates are thought to have evolved. However it is during embryonic development that the first differences between the vertebrate chordates and the protochordates can be seen. Vertebrate chordates have duplication of the Hox gene complexes during embryonic development, with which there is a relationship between the number of duplications and the complexity of the organisms anatomy; the greater the duplication the more complex the anatomy. There is also development of a neural crest that does not occur in protochordates. Some of the most important differences between vertebrate chordates and protochordates are in there skeleton and nervous systems. Vertebrate chordates have a more developed Triparhite brain surrounded by a cranium compared to the visceral cerebrium of the protochordates, and the head of vertebrate chordates extends beyond the notochord rather than the notochord extending to the tip of the head. The notochord still provides the majority of support in primitive vertebrates such as lampreys and Hagfish however in lampreys and more developed vertebrates there is also a vertebral colomn present around the dorsal hollow nerve cord from which the name of the subclass stems. There is finer tuning of the sensory organs of vertebrate chordates in comparison to their protochordate counterparts, with multicellular sense organs such as eyes and ears compared to the photoreceptors of protochordates and also vertebrate chordates have improved distance sensations with a lateral line unlike protochordates. Further to this vertebrate chordates have an electroreception system [depends on group] which is not present in protochordates and this system is very important in the Squalomorpha and Galeomorpha (shark groups) in catching and detecting prey. Numerous differences are also found between the respiratory and gut systems of protochordates and vertebrate chordates. Vertebrate chordates have gill bars [depends on group] whose function is support of the respiratory gills and not in filter feeding. The gills themselves are more complex containing gill filaments and are less numerous than in protochordates; with only 6-10 pairs compared to the hundreds of protochordates. The gill bars are not made of collagen but instead cartilage and have elastic recoil which aids the muscularised ventilation of the gills, another defining feature of vertebrate chordates.

4 The oxygen that diffuses into the gills of vertebrate chordates is transported in the haemoglobin of the red blood cells, these are absent in the protochordates. Vertebrate chordates have a muscularised gut in which food travels via peristalsis rather than ciliary action like in protochordates. They also have distinct liver and pancreas tissue and a glomerular kidney compared to the more primitive flame cells of protochordates. Hagfish (Myxiniformes) are one of the most primitive vertebrate groups and many of the features of vertebrates mentioned here are not present or less well developed in Hagfish. Some of the important exceptions are the absence of a vertebral colomn, electroreception, neural control of the heart and the poor development of the lateral line system. In contrast many of the more contempory species of vertebrates have secondary loss of features [which features?] and also the presence of certain features such as the notochord only in transitionary embryonic development. The differences between vertebrates is a result of the existence adaptive radiation that they have experienced to inhabit nearly all of the habitats in the world from mountain tops to deep sea vents. Generally well organized. Very good introductory paragraph, but lacks a concluding paragraph. Good understanding of subject, but depth of coverage is variable. This is a predominantly accurate and well written answer with some good use of examples. However, it falls short of a first class mark because of a couple of omission: (i) all vertebrates were grouped together (and some primitive characters and some other features are not seen in all vertebrates); (ii) there was limited discussion of bone. Mark = 68

5 APS214 (Vertebrates) Question 4 Discuss the features that make the vertebrate chordates unique from the invertebrate chordates. In the phylogenetic tree of the animal kingdom the closest relatives of the chordates are the echinoderms and the hemichordates. The chordates are distinguished from these (and otyher invertebrate) groups by the presence of a true notochord, the hollow dorsal nerve cord, pharangeal pouches or silts, a post-anal tail, and finally a thyroid gland (homologous to the endostyle that is used for filter feeding in inertebrates). There are three sub-classes of chordate two of which are invertebrate (protochordates) and one of which is vertebrates. The aim of this essay is to discuss the main features of the vertebrate chordates that distinguish them from the protochordates, and also to include a discussion of exceptions to the general features. The main unique feature is the presence of a vertebral column, consisting of bare, or in some cases, cartilaginous vertebrae. Bone is unique to vertebrates too though not all vertebrates have bone, for example Hagfishes. It originated when Calcium carbonate and phosphate were decomposed in a matrix and laid down in collagen. It is strong and has many advantages, such as support and protection. Not all vertebrates have vertebrae, for example, hagfishes. However, all vertebrates do have a cranium (brain case). This is a part of the skull, constructed from many flat bone plates fusing together, and it protects the brain, which is uniquely tripartile in vertebrates, vertebrates also possess multicellular sense organs, such as the eye and inner ear which are controlled by the brain. Vertebrate possess a chambered, pumping heart and have a closed circulatory system, unlike invertebrate chordata, with an extensive capillary network. The circulatory system is neurally controlled, and, unlike invertebrate chordata, the blood is directly involved in the transport of respiratory gasses. Red blood cells contain the respiratory pigment haemoglobin, which binds to oxygen and carbon dioxide and transports it via the capillary network. Vertebrates have a structure, or chamber, called the pharynx which links the mouth (buccal cavity) and oesophagus and trachea, through which, both food and respiratory gasses pass. The presence of food in the pharynx enduces swallowing, and the muscularised gut of vertebrate chordate moves the food along by peristalsis. Unlike invertebrate chordate digestion, which is intracellular, vertebrate digestion is intracellular, vertebrate digestion is extracellular. Enzymes are released onto the ingested substances which breaks the food down, and the parts are then taken into the cells. Vertebrates also have kidneys, which are dorsally located and who s function is to remove waste through ultrafiltration. They then release the waste via ducts which lead to the epidermis through the clonca. Kidneys also play an important role in ion concentration regulation. It should also be noted that all vertebrates, except hagfishes, are hypotonic to marine water, so regulation of ion concentrations is very important in marine vertebrates.

6 There are also embryological features which make vertebrate chordate unique from invertebrate chordate, for example the presence of neural crest, from which developes the front part of the head. Another being the duplication of the hox gene complex. This is a series of nucleotides which encode a set of amino acids called the homeodomain. This important for the binding of certain proteins to the DNA molecule. In conclusion, it is apparent that the vertebrate chordates evolved from invertebrate chordate ancestors. Thus both groups have many characters in common that the vertebrate chordates inherited from the invertebrate chordates. However, the vertebrate chordates also have many novel and distinguishing characters (both anatomical and physiological) that are unique to this group that evolved during the evolutionary transition from the invertebrate to vertebrate chordates. These characters define this group. This answer is brief but is factually accurate and well constructed. It demonstrates clear evidence of the ability to evaluate and synthesize information. The essay structure highlights similarities and differences by making comparisons explicit. The essay would have scored higher in the first class category if the facts had been expanded upon and there had been evidence of external reading. Well written (logical) with good use of examples. Excellent introduction and concluding paragraph. Mark = 72