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An in-depth exploration of various subphylums and classes within the chordate family, from the sessile filter feeders known as sea squirts to the jawed fish and eventual emergence of birds. Key topics include the notochord, somites, and the evolution of vertebrates.
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The urochordates include the tunicates, a.k.a., sea squirts
The tunicates are mostly sessile, filter feeding animals that look almost nothing like a chordate
However, their larval form possesses all of the basic characteristics of a chordate
The notochord serves as a primitive internal support structure
The notochord is a stiffened but still flexible rod found between the ventral (front/bottom) gut and the dorsal (back/top) nerve cord
Subphylum Cephalochordata
Cephalochordates, the lancelets, retain a chordate body plan throughout life
In addition to the four defining chordate characteristics, lancelets, as well as tunicate larvae, have somites
Somites are blocks of musculature arranged in segments along the bodies of lancelets and fishes (as well as derivations of this segmentation found in tetrapods)
Class Cephalaspidomorphi
Class cephalaspidomorphi is represented in extant lineages by lampreys
Lampreys lack jaws, like their non-vertebrate, chordate ancestors and hence are known as jawless fish
Jawless fish were the vertebrate forerunners of the jawed fish
Lampreys in addition lack paired fins
Lamprey have vertebrae (backbones) but no jaw
As vertebrates, lampreys are (just barely) members of subphylum Vertebrata Docsity.com
Jawed fish are referred to as gnathostomes (superclass gnathsomata) for their defining feature: jaws
Jawed fish also have skulls, vertebrae, and paired appendages (fins)
Vertebrate characteristics include a head, vertebrae, endoskeleton, and a closed circulatory system Many vertebrate characteristics are adaptations to vigorous swimming
Most also have ribs, jaws, and paired appendages (fins, legs)
Jaws, of course, aid in eating
Especially carnivorous eating…
Osteichthyans are Bony Fish
The bony fish are all of the fish you are familiar with except the sharks, rays, and chimaeras The skeleton of bony fish displays ossification, i.e., calcium phosphate in addition to the cartilaginous base Bony fish additionally possess flattened scales (unlike those of sharks) Bony fish also secrete mucus onto their skin to aid in reducing their coefficient of drag Bony fish additionally possess a gas-filled swim bladder that allows them to adjust their buoyancy to match that of the water
Lobe fins are supported by musculature and a bony skeleton; they may be employed for “walking” upon the bottom and other substrate found within bodies of water
The tetrapods are the terrestrial descendants of some lobe- finned fish, starting approximately with the amphibians
The amniotic egg is shelled, an adaptation to desiccation prevention
The amniotic egg employs extraembryonic membranes to transfer stored nutrients and water, exchange gasses, and remove wastes