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PRESENTED AND PREPARED BY SADIA KHURSHID LECTURER ZOOLOGY DEPARTMENT DA COLLEGE FOR WOMEN,PHASE VIII
PHYLUM CHORDATA (Protochordates) INTRODUCTION, CLASSIFICATION AND GENERAL CHARACTERS OF CHORDATES AND PROTOCHORDATES
OUTLINE Definition Fundamental characters General characters Classification of chordata Group Acraniata UROCHORDATA More details Systems CEPHALOCHORDATA SYSTEM
DEFINATION The name of this phylum is derived from two Greek words, the chorde meaning a string or cord, and ata meaning bearing, group or forms. Thus chordates are animals having a cord, i.e., notochord.
CHORDATES They include about 45000species including many animals of major economic importance. Phylum Chordata includes the most highly evolved animals, the vertebrates, as well as the marine invertebrate cephalochordate, amphioxus and tunicates.
FUNDAMENTAL CHARACTERISTICS OF CHORDATES All chordates shows all or at least any one of the following three fundamental characteristics: Notochord Nerve cord Pharyngeal gill slits
1-NOTOCHORD(chorda dorsalis) DEFINATION: The notochord is an elongated, flexible cartilaginous rod-like, skeletal structure lies dorsal to the gut tube and ventral to the nerve cord.
DIAGRAM OF NOTOCHORD
Fates of notochord In most adult chordates the notochord disappears or becomes highly modified, or it is surrounded and replaced by a vertebral column. In some non-vertebrate chordates and fishes the notochord persists as a laterally flexible but incompressible skeletal rod.
FIGURE 1: A generalized chordate showing fundamental chordate characters
2-NERVE CORD DEFINATION: The nerve cord of chordates develops dorsally in the body as longitudinal, a fluid filled hollow tube, lying just above the notochord and extending lengthwise in the body.
Fates of nerve cord In most species it differentiates in embryogeny into the brain anteriorly and spinal cord that runs through the trunk and tail. Together the brain and spinal cord are the central nervous system to which peripheral sensory and motor nerves connect.
3-PHARYNGEAL GILL SLITS DEFINATION: In all chordates, at some stages of their life history, a series of paired lateral gill cleft or gill slits perforate through the pharyngeal wall of the gut behind the mouth. These are variously termed as branchial, visceral or pharyngeal cleft or pouches. The soft and skeletal tissues between adjacent clefts are the visceral arches. (FIGURE. 1)
Function of gill cleft Gill cleft serve primarily for the passage of water from the pharynx to outside, thus bathing the gills for respiration. The water current secondarily aid in filter feeding by retaining food particles in the pharynx.
Embryonic fate of the clefts Non-vertebrate chordates: The clefts and arches are elaborated as straining devices concerned with capture of small food particles from water and are function throughout life. Fish-like vertebrates and juvenile amphibians : The walls of the pharyngeal clefts develop into gills that are organs of gas exchange between the water and blood. Adult amphibians and the amniotes tetrapods: The anterior most cleft transforms into the auditory tube and middle ear chamber, whereas the other clefts disappear after making some important contributions to glands lymphatic tissue in the throat region.
GENERAL CHARACTERISTICS OF CHORDATES Chordates in general have following characteristics: Bilateral symmetry Triploblastic Deuterostome Complete digestive tract Well developed coelom Internal organs are suspended in the coelomic cavity by a thin membranous tissue called mesentery Sexual reproduction Post anal tail
CLASSIFICATION OF CHORDATES PHYLUM CHORDATA GROUP 1 ACRANIATA SUB –PHYLUM UROCHORDATA SUB- PHYLUM CEPHALOCHORDATA GROUP 2 CRANIATA SUB-PHYLUM AGNATHA SUB-PHYLUM GNATHOSTOMATA
1-Group ACRANIATA (Protochordata) They are the simple or first chordates in which brain box (cranium) is absent and hence brain is not prominent. Notochord does not transform into vertebral column. Group acraniata is divided into two sub phylum: Urochordata (notochord in the tail) Sub-phylum cepalochordata (notochord from head to tail)
1-SUB-PHYLUM UROCHORDATA (Tunicata) Urochordates have a notochord that extends from just behind the head to the tail (rather than from head to tail; Urochordata means "tail- cord"). Urochordates are also called tunicate because their body is enclosed in a sac called tunic or test composed largely of tunicine, similar to cellulose. (FIGURE-2) Tunicate includes about 2,000 fixed and nearly 100 pelagic species.
FIGURE: 2 Tunicates: sea squirts
More about Urochordates DIGESTIVE SYSTEM:They are all marine and sessile. Most tunicates feed by filtering sea water through pharyngeal slits.Tunicates are suspension feeders. They have two openings in their body cavity, an in-current and an ex-current siphon. The in- current siphon is used to intake food and water, and the ex-current siphon expels waste and water. The tunicate's primary food source is plankton. Plankton gets entangled in the mucus secreted from the endostyle.The tunicate's pharynx is covered by miniature hairs called ciliate cells which allow the consumed plankton to pass down through to the esophagus. Their gut is U-shaped, and their anuses empty directly to the outside environment. They have complete digestive tract.(FIGURE-3)
Figure 3: Internal organs of adult ascidians (sea squirt). Large arrows show water flow through the animal; small arrows, the path food trapped by the pharyngeal gill slits takes through the animal.
RESPIRATION, EXCRETION AND COELOM RESPIRATION: They have two to seven pairs of gill slits. Respiration through test and gill slits. EXCRETION: Tunicates lack the kidney-like metanephridial organs. They have neural gland, pyloric gland and nephrocytes. COELOM: The original coelom body-cavity develops into a pericardial cavity and gonads.
What the Urohordates have? Like other chordates, tunicates have a notochord during their early development, but lack myomeric segmentation throughout the body and tail as adults. Blood vascular system:Blood vascular system is of open type. Heart is simple, ventral and tubular. Tunicate blood contains high concentrations of the metal vanadium,vanadium-associated proteins as well as lithium.
REPRODUCTION Most tunicates are hermaphrodites. Tunicates begin life in a mobile larval stage that resembles a tadpole, and exhibit chordates characters. It contains a nerve cord and a short notochord in its tail only. As the larva reaches maturity it attaches to the sea bottom and undergoes retrogressive metamorphosis by losing its tail and most of the chordate characters.only gill slits persist. Later, it develops into a barrel-like and usually sedentary adult form. (FIGURE-4) IN PAKISTAN: Many species of Herdmania are fond in our seas.
FIGURE 4: LARVAL AND ADULT ASCIDIAN Ascidian Larvae Young Ascidian Larvae Older Ascidian Larvae Adult Ascidian
2-SUB-PHYLUM CEPHALOCHORDATA DEFINATION: Cephalochordates are small, eel- like animals that spend much of their time buried in sand. They have notochord and nerve cord extends from head up to the tail, and persists throughout life.
Chordates features Cephalochordates have all the typical chordate features. dorsal nerve cord notochord. pharyngeal slits (100 )which are used to strain food particles out of the water. The musculature of the body is divided up into V-shaped blocks, or myomeres. post-anal tail. On the other hand, cephalochordates lack features found in most or all true vertebrates, small brain and poorly developed sense organs and no true vertebrae. (FIGURE-5)
FIGURE 5: AMPHIOXOUS
DIGESTIVE SYSTEM: Digestive tract is complete. Pharynx is large perforated by numerous gill slits. They are filter feeder. A pouch or hepatic caecum secretes digestive enzymes, and actual digestion takes place in a specialized part of the intestine known as the iliocolonic ring. (FIGURE -6)
FIGURE 6: AMPHIOXOUS INTERNAL STRUCTURE
More about cephalochordates RESPIRATION: Respiration through the general body surface. No special organ for respiration. CIRCULATORY SYSTEM: Cephalochordates also have a well- developed circulatory system. EXCRETORY SYSTEM: Simple excretory system composed of paired nephridia.
REPRDUCTION Reproduction is sexual. The sexes are separate, and both males and females have multiple paired gonads. Eggs are fertilized externally, and develop into free-swimming, fishlike larvae. No asexual reproduction. Development is indirect.
SPECIES NUMBERS Cephalochordates comprises about 30 species. IN PAKISTAN: Out of only two genera found around the world, Branchiostoma is found on our coast.
REFERENCES WEBSITE USED: BOOKS: Elements of Chordate Anatomy by Charles K. Weichert Modern Text Book of Zoology, Vertebrates. R.L. Kotpal