Protochordata ZLY 304 Phylogenetic Relationship between Echinoderms Hemichordates

Protochordata ZLY 304

Phylogenetic Relationship between Echinoderms, Hemichordates & Chordates

Compound Tunicate Colony

Salps

Doliolum

Amphioxus

Protochordata • Proto = first, literally means 'the first chordates’. • heterogeneous group of animals of phylum Chordata, related to the vertebrates • The Vertebrata and the Protochordates share the following ü hollow dorsal nerve cord ü gill slits ü notochord • The protochorda differ chiefly from the vertebrates in not having a backbone. • Deuterostomes Tunicate Colony Ciona

General characteristics of Protochordata • Exclusively marine & rrelatively small • No cranium or brain box, no jaws, no vertebral column and no paired appendages • Dorsal tubular nerve cord, gill slits and notochord are present • Sexes may be separate or united • Solitary, colonial, free living, pelagic, burrowing or tube like living forms • Three Sub phylums Ascidia

Class - Hemichordata (Acorn worms) • • • hemi = half; chorde = cord / short notochord, reduced to half the size Three body parts as proboscis, collar, and trunk Notochord occurs only in the proboscis The dorsal nerve cord in the collar and trunk resembles the nerve cord of chordates. The pharynx below the collar has numerous gill slits about 90 species living on tidal mud flats & exclusively marine Fertilization is external Larva resembles the larva of echinoderms They include three groups – Enteropneusta – Balanoglossus, Saccoglossus – Pterobranchia-Cephalodiscus

Class I- Enteropneusta • Solitary and burrowing worm-like marine forms commonly known as acorn or tongue worms • Live close to 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 – Balanoglossus – Saccoglossus

Class II- Pterobranchia • 20 species of Sedentary, solitary or colonial, bottom dwelling marine worms • 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 • Pterobranchia includes • Reproduction by budding – Cephalodiscus

Subphylum Urochordata (Tunicate/sea squirts) • uro=a tail; chorde=cord • Solitary or colonial and exclusively marine forms • Three classes – Larvacea – Ascidiacea – Thaliacea • contains 1, 250 species • notochord is present in the tail only of the larval forms • Adults have a body composed of an outer cuticular tunic with an incurrent and excurrent siphon Solitary Ascidians

Subphylum Urochordata (Tunicate/sea squirts) • Dorsal tubular nerve cord is present in the larva & degenerates to a small ganglion in adults • Gills are the only chordate character that remains in adults • Beating of cillia creates a current to move • squirt water out when they are disturbed • Food particles adhere to a mucous secretion of the cilia lining in the pharynx • bilaterally symmetrical larvae has three chordate characteristics • larva metamorphoses into the sessile adult • Sexes are united that is hermaphrodite.

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 – Oikopleura Larvacian Oikopleura

Class Larvacea

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 • Ciona • Ascidian

Ascidia

Ciona

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. Thaliacian • Life history exhibits an alternation of generations. • Doliolum

Doliolum

Subphylum Cephalochordata lancelets • cephalon – head; chorde – cord • notochord extends from head to tail • notochord lies on the mid dorsal region just above the alimentary canal and below the nerve cord. • Notochord, nerve cord and pharyngeal gill slits remain throughout life of the animal. • Exclusively marine and solitary forms • No limbs or paired fins • Mouth is ventral and anterior, while anus is ventral and posterior. • No distinct head but tail present; mouth surrounded by tentacles

Branchiostoma- Amphioxus • 23 species of lancelets are in the genus Branchiostoma • elongated, lance-shaped = lancelet = a two-edged surgical knife • inhabit shallow coastal waters • partly buried in sandy substrates and filter feed • Lancelets feed on microscopic particles filtered from the constant stream of water that enters the mouth and exits through the gill slits into an atrium that opens at the atriopore. • Lancelets have segmented muscles and their dorsal hollow nerve cord has periodic branches • Exoskeleton, head, brain, auditory organs and jaws are absent

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.

• Describe the characteristic feature of the three sub phylum of protochordata • Describe the feeding adaptations of the three sub phylum of protochordata GOODLUCK DR. WASANTHA SENA WELIANGE wasantha. weliange@yahoo. com