Vertebrate Zoo L2: The Diversity of Craniates and Characteristics of the group

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  • The Diversity of Craniates and Characteristics of the group
    • Diagnostic features of craniates
      • Skull (Craniata)
        • An elaborate skull is a feature shared by lampreys and jawed vertebrates (the hagfish skull is simpler in construction)
        • Neurocranium is the innermost component (immediately surrounds brain and sense organs)
          • Originates as cartilage around the brain and sense organs- second oldest component of the skull
          • Consists of 4 components
            • Bones of occipital region
            • Bones of temporal region
            • Bones of sphenoid region
            • Bones of the ethmoid region
          • Present in embryos of all craniates, and represents the adult condition in hagfish, lampreys and cartilaginous fish
            • Neurocranium of an adult shark is a single element, called a basal plate, deriving from fused cartilages.
              • Pairs of capsules attached, which support and protect organs of hearing, sight and smell
        • Dermatocranium is everything but the face, and forms the external sheath around the neurocranium
          • Develops from dermal bone (phylogenetically ancient constituent)
            • Dermal bones formed through the deposition of bone tissue within the connective tissue
            • Consists of 4 compoents
              • 1. Dermal bones of cranial roof
              • 2. Dermal bones of the primary palate
              • 3. Dermal bones of the upper jaw
              • 4. Dermal bones of the lower jaw
            • Many primitive craniates had extensive body cover consisting of dermal plates, but dermatocranium is most recently evolved component of the skull
          • Absent only in hagfish, lampreys and cartilagious fish
        • Splanchnocranium (AKA Viscerocranium)
          • Originates from pharyngeal arches (oldest part of the skull)
            • support and move gills and formation of jaw
          • Present in all embryos, represents adult condition in hagfish,lampreys, cartilaginous and bony fish
          • Consists of 3 components
            • 1. First arch (mandibular arch)
              • Two segments only, unlike others made of several segments
            • Second arch (hyoid)
            • Additional arches (3rd up to 6th or 7th)
      • Sensory placodes
        • Localised thickenings in the ectoderm of the head region of a craniate's embryo
        • Olfactory epithelium lining the nasal cavities develops from an olfactory placode... eye lens from lens placode... inner ear from otic placode...
          • Formation: e.g. invaginating placode then otic pit to otic vesicle
        • In aquatic vertebrates, the lateral line placodes give rise to the lateral line system
      • Five-part brain
        • Brain of craniates develop from five vesicles of the neural tube
        • Craniates share basic modalities of brain development
      • Cranial nerves
        • Paired cranial nerves
          • III, IV and VI only in vertebrates
      • Nasophypophysis
        • Accomodates olfactory organ and pituitary gland
        • Primitively, has anterior opening, or external nostril (Anteriorly on snout (hagfish) or atop head (lamhreys)
        • In hagfish, the single nostril can be large while in lampreys it is usually very small
        • Lampreys and hagfish: single nostril continues as a hypophyseal duct that leads to nasal capsule (olfactory organ) situated in a median position
        • Jawed vertebrates show two external nostrils, leading to two separate nasal capsule
          • Position of capsules is more anterior in position, anterior or anterodorsal to the mouth
            • In hagfish and lamprey, the single capsule occurs dorsal to the pharynx
      • Semicircular canals
        • Detect angular acceleration and deceleration of the head.
        • 3 in jawed verts, 2 in lampreys and 1 in hagfish
        • Each canal detect motion in a singular plane- activated by circular motion
          • filled with endolymph- motion of fluid informs brain about direction of movement
      • Gill filaments
      • Caudal fin skeletons
        • supported by cartilaginous rods in a hagfish embryo
      • Endoskeleton
        • Cartilaginous and / or bony
      • Complex digestive and circulatory systems
        • Double circulation in mammals
        • Increasing complexity of the heart of craniates
      • Neural Crest
        • Temporary group of cells unique to craniates that arise  from the embryonic ectoderm cell layer, and in turn give rise to  diverse structures.
          • Sensory neurons
          • Some skeletal and connective tissues in skull
          • Some pigment-containing cells and other integumentary tissues
          • Gill arches, jaws, and parts of the braincase floor
          • Jawed verts: involved in formation of dermal skeleton (scales, teeth and dermal bones)

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