What are protochordata?Why are protochordata are primitive vertebratePresenters; Shree jana Bomjan Pema Tshojay Kichu Lhaden
Phylum-Chordata Subphyla Protochordata( Acrania) Subphyla Vertebrata (Craniata) hemichordata Cephalochordata Urochordata Larvacea Ascidiacea Thaliacea Enteropneusta Pterobranchia
Protochordata The name Protochordates literally means 'the first chordates’. Protochordata - a heterogeneousgroup of animals of phylum Chordata, related to the vertebrates, which they resemble in possessing gill slits, noto-chord, and dorsal hollow nerve cord, or at least traces of these. Protochordates are marine animals`
Why are protochordates not classified as true Chordata • Like the remaining subphylum of the chordates, the Vertebrata, the protochordates have a hollow dorsal nerve cord, gill slits, and a stiff supporting rod, the notochord, the forerunner of the backbone. • The protochordates differ chiefly from the vertebrates in not having a backbone. • Recent protochordates are thought to have evolved from the same ancestral stock as that which gave rise to the vertebrates. http://wiki.answers.com/Q/Why_are_protochordates_not_classified_as_true_chordates#ixzz1MathrUsV
General characteristics of protochordata • Exclusively marine. • Relatively small sized animals • Cranium or brain box, jaws, vertebral column and paired appendages are absent. • Dorsal tubular nerve cord, gill slits and notochord are usually present. • Sexes may be separate or united. • Solitary, colonial, free living, pelagic, burrowing or tube like living forms (A manual of practical zoology Chordates by P. S. Verma)
1) Hemichordata The name Hemichordata refers to the presence of a short notochord, reduced to half the size (hemi – half; chorde – cord). This structure is present in the anterior region of the animal, the proboscis They include two groups; • Enteropneusta • Pterobranchia
General Characteristics of hemichordates • Exclusively marine and soft-bodied forms • Body is divisible into proboscis, collar and trunk • Notochord occurs only in the anterior end of the body. • Numerous paired gill slits are present. • Nervous tissues lie embedded in the epidermis and occur both on the dorsal and ventral surfaces. • Coelom is divided into three regions namely protocoel, mesocoel and metacoel. • Fertilization is external.
Class I- Enteropneusta • Solitary and burrowing worm-like marine forms commonly known as acorn or tongue worms. • vegetation; filter-feeders • They have well-developed gill slits • Epidermis is ciliated and glandular. • Alimentary canal is straight with a terminal anus. • Two rows of caecae are present in the middle of the trunk. • They have a dorsal strand of nerve cells ( A manual of Practical Zoology Chordates by P.S Verma)
Examples of Enteropneusta Balanoglossus Saccoglossus
Class II- Pterobranchia • Sedentary, solitary or colonial and marine forms. • Bears a ciliated tentacles to produce a Ciliary feeding currents of water. • One pair of gill slits or none. • There is no trace of dorsal nerve cord or notochord • Alimentary canal is U-shape with dorsal anus situated near the mouth. • Gonads are few in numbers. • Reproduction by budding. ( A manual of Practical Zoology Chordates by P.S Verma) )
2) Urochordata • Urochordata is the term used to refer to the presence of a notochord in the tail region, (uro=a tail; chorde=cord). • The notochord is restricted to the tail region of the larval forms of urochordates and is absent in the adults. • Tunicata is the other name of this subphylum Urochordata, due to the presence of an outer leathery covering called tunic or test in the adult (tunica – outer covering).
General characteristics of Urochordata • Exclusively marine and commonly known as sea squirts. • Solitary or colonial • Body is covered by a cuticular tunics or test in adult stage. • Notochord is present in larval stages and absent in adults. • Dorsal tubular nerve cord is present in the larval forms while degenerates in the form of small ganglion in adults. • A numerous gill slits are present. • Sexes are united that is hermaphrodite. • Heart is ventral, simple and tubular.
Class I- larvacea • Free swimming pelagic forms • Neotenic forms which retain the larval form throughout adult life. • Posterior part of the body takes the form of a large locomotory appendage, the tail. • Single pair of gill slits are present • Anus opens ventrally on the surface of the body • Hermaphrodite • No metamorphosis. ( A manual of Practical Zoology Chordates by P.S Verma)
Examples of Urochordata Oikopleura
Class II- Ascidiacea • Fixed or free swimming marine forms • Simple or compound, solitary or colonial. • Locomotory appendage or tail are absent in adults • No traces of notochord • Branchial sac is large and well developed with its walls perforated by numerous gill slits. • Reproduction is both asexual and sexual ( A manual of Practical Zoology Chordates by P.S Verma)
Class III- Thaliacea • Free swimming pelagic forms. • Solitary or colonial • Musculature of the body wall is in the form of circular bands. • Branchial sac has either two large or many small gill slits. • Tail and notochord are absent in adult. • Life history exhibits an alternation of generations. ( A manual of Practical Zoology Chordates by P.S Verma)
Cephalochordata • The term Cephalochordata refers to the notochord that extends the entire length of the body up to the head region (cephalon – head; chorde – cord). • The notochord lies on the mid dorsal region just above the alimentary canal and below the nerve cord.
General characteristics • Exclusively marine and solitary forms • Notochord and nerve cord extend the entire length of the body. • Notochord, nerve cord and pharyngeal gill slits remain throughout life of the animal. • Limbs or paired fins are absent. • Mouth is ventral and anterior, while anus is ventral and posterior. • No distinct head but tail present; mouth surrounded by tentacles • Exoskeleton, head, brain, auditory organs and jaws are absent. • Sexes are separate. ( A manual of Practical Zoology Chordates by P.S Verma)
Why are the protochordates so primitive • Chordates showing primitive features are collectively grouped as protochordates. • have a notochord, which is an elongate, stiff, fleshy structure running down the length of the body, giving structure to the body, enough so that muscles can fire down the body enechelon to form the familiar "wiggly" swimming motion of fish. • The notochord is modified into the vertebral column in vertebrate chordates.
The nervous system is of a primitive nature, lying beneath the body wall. Including a dorsal (toward the top of the body) nerve cord, which is known as the spinal cord. • Formed in the embryo by an invagination of surface ectoderm whose original function was probably sensory reception.
The ancestor was a fishlike deuterostome ( appears to form a natural evolutionary line in which there are common features as well as similarity in embryonic development) that swam using alternating contractions of right and left longitudinal axial muscles to create undulations of the body. • Paired pharyngeal gill slits connect the lumen of the pharynx with the exterior and originally functioned in suspension feeding with respiration being added later. • (http://lander.edu/rsfox/invertebrates/branchiostoma.htm/)
Reference list P.S. Verma. (1984). A Manual of Practical Zoology Chordates. New Delhi: S. Chand and Company LTD. Retrieved on 10th of May, 2011 from http://www.google.com/search?um=1&hl=en&biw=1073&bih=410&tbm=isch&sa =1&q=balanoglossus&aq=0&aqi=g4&aql=&oq=Balanoglossus http://www.britannica.com/EBchecked/topic/480249/protochordate http://wiki.answers.com/Q/What_is_protochordata#ixzz1MgWugpPV http://wiki.answers.com/Q/Why_are_protochordates_not_classified_as_true_chordates#ixzz1MathrUsV