Presentation on theme: "Introduction to the Phylum Chordata"— Presentation transcript:
1 Introduction to the Phylum Chordata Honors Biology
2 Unifying Themes1. Chordate evolution is a history of innovations that is built upon major invertebrate traitsThey display many of the basic traits that first evolved in the invertebrates: bilateral symmetry, cephalization, segmentation, coelom, "gut" tube, etc.2. Chordate evolution is marked by physical and behavioral specializationsFor example the forelimb of mammals has a wide range of structural variation, specialized by natural selection3. Evolutionary innovations and specializations led to adaptive radiations - the development of a variety of forms from a single ancestral group
3 Characteristics of the Chordates Notochord, dorsal hollow nerve cord, pharyngeal gill slits, blocks of muscle, postanal tail
4 Characteristics of the Chordates The notochordAll chordate embryos have a notochord, a stiff but flexible rod that provides internal supportRemains throughout the life history of most invertebrate chordates; among, present only in the embryos of vertebrate chordates
5 Dorsal Hollow Nerve Cord (=Spinal Cord) Characteristics of the Chordates cont.Dorsal Hollow Nerve Cord (=Spinal Cord)A fluid-filled tube of nerve tissue that runs the length of the animal, dorsal to the notochordPresent in chordates throughout embryonic and adult life
6 Pairs of opening through the pharynx Characteristics of the Chordates cont.Pharyngeal gill slitsPairs of opening through the pharynxInvertebrate chordates use them to filter foodJuvenile fishes use them to them for breathingIn adult fishes the gill sits develop into true gillsIn reptiles, birds, and mammals the gill slits are vestiges, occurring only in the embryo
7 Blocks of Muscle - Myotomes Characteristics of the Chordates cont.Blocks of Muscle - MyotomesSurrounding the notochord and nerve cord are blocks of muscle - myotomesPostanal TailThe notochord, nerve cord, and the myotomes extend to the tailFound at some time during a chordate's development
9 SubPhylum Urochordata Marine animals; some species are solitary, others are colonial.Sessile as adults, but motile during the larval stagesPossess all 5 chordate characteristics as larvaeSettle head first on hard substrates and undergo a dramatic metamorphosis (e.g., tail, notochord, muscle segments, and nerve cord disappear)
10 SubPhylum Urochordata cont. Adult body is covered by an outer envelope or tunic; composed of fibers of tunicin embedded in a mucopolysaccharide matrixTunic encloses a basket-like pharynx, that is perforated by gill slitsTunicates are filter feeders; plankton is trapped in a sheet of mucus and cilia later direct the food-laden mucus to the stomachWater leaves the animal via an excurrent siphon
11 Chordate MetamerismBody segmentation (i.e. metamerism) appears to have evolved in two lineages of the chordates: the Cephalochordates and the Vertebrates; probably occurred after divergence from the UrochordatesHowever, segmentation in the chordates does not involve the coelomThe cephalochordates and the chordates movement is accomplished by contraction of muscle fibers that are arranged in segmented blocks - myotomesPresumably, segmentation of muscles developed as an adaptation for undulatory swimming and rapid burrowing
12 SubPhylum Cephalochordata Exclusively marine animalsAlthough they are capable of swimming, they usually are buried in the sand with only their anterior end being exposed
13 All chordate characteristics are present throughout their life history SubPhylum Cephalochordata cont.All chordate characteristics are present throughout their life historyThey are filter feeders: inside of the oral hood is lined with cilia -wheel organThese cilia, plus cilia in the pharynx help generate a water currentWater and suspended food particles pass through the oral hood, equipped with projections called cirri that strain larger particlesFeed by secreting a mucous net across the gill slits to filter out food particles that are present in the water.
15 General Characteristics Exhibit all 5 chordate characteristics at sometime in their life historyUsually well cephalized, including a well developed brain and a number of anterior sensory structuresBrain is usually encased in a skull, made of hard bone or a cartilage.In most vertebrates, the embryonic notochord is replaced by a vertebral column.Possess a distinctive endoskeleton consisting of vertebral column, limb girdles, two pairs of jointed appendages, and a head skeletonMuscles are attached to the skeleton to provide movementOften have a muscular perforated pharynxClosed circulatory system with a well developed muscular heart; blood is oxygenated as it flows through vascularized skin, gills or lungs.
16 Evolutionary Relationships of the Vertebrates Earliest vertebrate fossils (jawless ostracoderm fishes; 500 mya) share many of the novel structures observed in the living vertebratesQ. When and from where did these vertebrate characteristics evolve?May have evolved from an invertebrate chordate lineageThis idea is supported by the discovery of a fossilized mid-Cambrain invertebrate chordate from the Burgess Shale formation - PikaiaA ribbon shaped, somewhat fish-like creature about 5 cm in lengthIt possessed a notochord and the V-shaped myomeresResembles Amphioxus, and may very well be an early cephalochordate
17 Evolutionary Relationships of the Vertebrates cont. Speculations regarding vertebrate ancestry have focused on living cephalochordates and tunicatesOne hypothesis on the evolution of the vertebrates is Garstang's HypothesisIt suggests that sessile tunicates were an ancestral stock that evolved a motile larval stageGarstang speculated that at some point larvae failed to metamorphose into an adult, but developed gonads and reproduced in the larval stageAnd with continued larval evolution a new group of free swimming animals evolvedGarstang called this process paedomorphosis, a term that describes the presence (or evolutionary retention) of juvenile or larval traits in the adult body