chordates - Chordates (44,000 species) All chordates share...

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Chordates (44,000 species) All chordates share the following [Fig. 34.3, p. 699]: - dorsal hollow nerve chord that forms spinal cord and brain - notochord at some stage of life cycle (forms intervertebral disks in more advanced groups) - pharyngeal gill slits at some point in life cycle - post-anal tail Several subphyla: Urochordata - tunicates - [Fig. 34.5, p. 701] - mostly marine, sessile - transitional circulatory system (mostly closed) - young are free-swimming larva, adults are sessile. - some seem to be causing problems as invasive species in some coastal areas. Cephalochordata - lancelets - [Fig. 34.4, p. 700] - mobile, though weak swimmers - blade like shape (hence name) - transitional circulatory system (mostly closed) Vertebrata - true vertebrates (fish, birds, mammals, etc.) - well developed nervous and sensory systems - skull, brain - closed circulatory system
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VERTEBRATES: I. Class Agnatha - jawless fish (sometimes considered a Superclass) - the book is totally nuts with its classification here. Use what is presented here. - surprise - no jaws! - lack paired fins - lampreys and hagfish - eel like shape - lampreys are parasitic - like huge leeches. Wiped out fishing in the great lakes when the St. Lawrence canal was opened. - toothed, sucker like mouth [Fig. 34.10, p. 704] : - attach to fish, and suck out body fluids. - hagfish are mostly scavengers [Fig. 34.9, p. 703] . - will bury into dead or sick fish and eat it out from the inside. - also make lots of slime when threatened. - don't really have a vertebral column (lampreys do). Jaws - possibly evolved from gill supports (gill arches) [Fig. 34.13, p. 706] II. Class Chondrichthyes - sharks, rays, etc. (chondros = cartilage, ichthys = fish) - skeleton is composed of cartilage rather than bone - no swim bladder. Unless “resting” must swim constantly. - 5 - 7 gill slits (one for each gill) - teeth are replaced continuously (that’s why shark’s teeth are so common). - most sharks are carnivorous (!); the the largest all feed on plankton (strain food over gills) - rays and skates live along the bottom, feed on mollusks and other bottom dwellers.
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- internal fertilization; some also have live birth (ovoviviparous) - areas in skin can detect electrical fields generated by muscle contraction (helps them zoom in on prey)! III. Class Osteichtyes (osteon = bone) - bony fish - the book once again makes a hash of things here. Stick with this classification. - bony fish - skeleton composed of bone
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chordates - Chordates (44,000 species) All chordates share...

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