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An overview of chordate animals, focusing on their shared traits: notochord, dorsal hollow nerve cord, pharyngeal gill slits, and a post-anal tail. It also covers various chordate subgroups, such as Hemichordata, Urochordata, Cephalochordata, and Vertebrata, and discusses their characteristics and adaptations.
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Typology: Lecture notes
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For any given chordate you should be able to identify to which major clade or taxon it belongs. You should also be able to describe important features of these animals.
The figure below illustrates the 4 traits that chordates have in common: notochord, dorsal hollow nerve cord, pharyngeal gill slits, and a post-anal tail. They do not necessarily possess all of these traits at all stages of their life cycle, but all chordates have these four traits at some point during their life cycle.
The Hemichordata are a small phylum that shares certain characteristics with the Chordata. Acorn worms have pharyngeal gill slits, and all Hemichordata have a dorsal, hollow nerve chord. Structures exist that may be homologous to the notochord, but these homologies are disputed.
The Hemichordata are now usually treated as a separate phylum closely related to the Chordata and to the Echinodermata. (Larval acorn worms and larval echinoderms are remarkably similar!) All Hemichordata are filter-feeders. In addition to the free-living acorn worms, the Hemichordata include the tube-building Pterobranchs, as well as an extinct group called Graptolites. Both pterobranchs and graptolites feed using feathery ciliated tentacles that resemble a lophophore. The acorn worms pass currents of water through their gill slits and use their gills as a feeding apparatus.
Examine and draw a preserved specimen of an acorn worm. Label the gill slits and the muscular proboscis used in burrowing.
Adult tunicates are sessile (attached) organisms that pump water through a seive-like gill basket with hundreds of gill slits. They filter feed by straining tiny food particles out of the water using this gill basket. Their actively swimming larval stage has a notochord, a dorsal hollow nerve chord, a series of segmentally organized swimming muscles, and a strong post- anal tail, but these larval features are lost or greatly diminished in the adult. Observe and draw an adult tunicate. Also, examine a slide of a larval tunicate if one is available.
The Agnatha are a paraphyletic group that contains the hagfishes (Myxini), lampreys (Petromyzontida), and several extinct groups. Examine preserved specimens of both lampreys and hagfish, locating the mouth and gill slits in each case. Examine the round mouth of the adult lamprey, with its embedded teeth.
The placoderms were the first Gnathostomes (fishes with jaws). They varied in size from a few inches long to terrifying creatures up to 30 feet (10 meters).
The Chondrichthyes have a skeleton made entirely of cartilage. Their skin contains thousands of small, embedded placoid scales. Most Chondrichthyes are predators. Their gill slits are externally visible, and they have no lungs or lung homologues.
The Osteichthyes, or bony fishes, include the vast majority of fish species alive today. Their sizes, shapes, and lifestyles vary greatly, but they all share two important traits:
When Tetrapods made the move onto land, they faced many of the same challenges that plants had when they made the big leap onto land.
What were some of these challenges? What adaptations permitted the transition from water to land? In what way are amphibians still to aquatic habitats? Why is the amniotic egg considered a key innovation?
The Amphibia are vertebrates with aquatic, gill-breathing larvae that undergo metamorphosis into lung-breathing adults. What adaptation do amphibians lack, that requires them to stay near a body of water? The posture of amphibian limbs is sprawling, with the body resting on its ventral surface when not in motion; this posture is best seen in salamanders (Urodela or Caudata). In addition to salamanders, modern amphibians include two other groups: the legless caecilians (Apoda) and the frogs and toads (Anura). The class Amphibia also includes a number of extinct groups that flourished in the Devonian through Triassic periods.
Order Apoda Order Urodela Order Anura
Reptiles were once a very diverse group, especially during the Mesozoic Era, but only a few reptile groups survive today.
Below is a family tree of dinosaurs, including the orders Ornithischia and Saurischia. We have many models on display in lab, and even more out in the hall. Notice that the group shown here is monophyletic and includes the birds. If the birds were excluded, the group would be paraphyletic. If we added various other large extinct reptiles (such as ichthyosaurs, plesiosaurs, and ancient crocodiles), such a group would be polyphyletic.
Birds:
Birds are warm-blooded vertebrates covered in feathers, and most of them fly. They were traditionally recognized as Class Aves, but, since they evolved from a group of saurischian dinosaurs, a growing number of zoologists now place them within the Class Reptilia. Examine several birds and notice their adaptations for flying.
Class Mammalia:
Mammals are warm-blooded vertebrates with a fully divided (four-chambered) heart that nourish their young with milk. Living mammals can be divided into a few egg-laying species (the Monotremes, including the platypus and echidna), the pouched mammals ( Marsupials, including opossums, kangaroos, koalas, and others), and the placental mammals ( Eutheria ).
Placental mammals (Eutheria)
The vast majority of mammals are placental mammals in which the young develop inside the mother's uterus and are born alive. While in the uterus, the young are nourished through a
placenta, an intimate connection of maternal and fetal blood vessels. Placental mammals include the following orders: