Protochordates

• A whole mount species of Ammocoete.
• The heart consists of a sinus, venosus, atrium and ventricle.
• Phylum Chordata was established in 1874 by Ernest Haeckel to incorporate three subphyla: Urochordata (sea squirts and certain other invertebrates), Cephalochordate (amphioxuses), and Vertebrata (chordates with vertebrate characteristics).
• Members of the two lower subphyla, Urochordata and Cephalochordata, are referred to as protochordates.
• All protochordates are marine organisms and it is believed that they and the vertebrates may have had common ancestors.
• The phylum Chordata takes its name from the notochord, a slender rod that develops from the mesoderm in all chordates.
• The pharynx is a part of the digestive tract located immediately posterior to the mouth and during some point in the lifetime of all chordates, the walls of the pharynx are pierced, or nearly pierced, by a longitudinal series of openings, the pharyngeal slits.
• The term gill slits are often used in place of pharyngeal slits for each of these openings, which serve primarily in feeding and have no significant role in respiration.
• The endostyle is a glandular groove in the floor of the pharynx involved in filter feeding and the thyroid gland, like the endostyle, is involved in iodine metabolism.
• The dorsal hollow nerve cord is derived from the ectoderm and formed by invagination.
• The postanal tail represents a posterior elongation of the body extending beyond the anus and is primarily an extension of the chordate locomotor apparatus, the segmental musculature and notochord.
• All protochordates are marine animals that feed by means of cilia and mucus, but they often live quite different lives as young larvae than they do as adults.
• As youngs they may be pelagic, residing in open water between the surface and the bottom, or they may be benthic, living on or within a bottom marine substrate.
• Some protochordates are free-floating larvae with limited locomotor capability and are therefore planktonic, riding from place to place primarily on currents and tides rather than by their own efforts of long-distance swimming.
• As adults, protochordates are usually benthic, living on or within a bottom marine substrate, some burrow into the substrate or are sessile and attached to it, and some are solitary, living alone; others are colonial and live together in associated groups.
• Some protochordates are dioecious, with male and female gonads in separate individuals, and others are monoecious, with both male and female gonads in one individual.
• The urochordates are chordates with notochord that is confined to the tail, also called tunicates because of the non-living tunic that covers them.
• The tunic is delicate in appearance, often beautifully colored and usually transparent, and covers the urochordates from head to tail.
• Urochordates vary in size from microscopic forms to several inches in length and exhibit a free-swimming larval stage, but only one group, ascidians, undergo complete metamorphosis.
• All urochordates are filter feeders, filtering food particles out of the incoming respiratory stream before the water passes the gills.
• Respiration results from passage of water containing oxygen from the pharynx through 100 or more gill slits on each side, past the gill bars that contain blood vessels, and into the atrium.
• In some species neither definitive blood cells nor a functional heart differentiates until metamorphosis.
• The notochord is the chief support of the body and is a slender rod of tall cells containing gelatinous material and is surrounded by a continuous sheath of connective tissue.
• The animal is now a filter feeder.
• Sea squirts are the common name for some of them because they discharge water when irritated.
• These folds were at one time considered to represent the beginning of paired appendages of vertebrates, a theory which is no longer tenable.
• On the cirri and tentacles and within the hood are sensory structures.
• The larvae of solitary sea squirts are tiny (0.5 mm to 11 mm), exist only for as little as few minutes, not longer than a few days.
• The circulatory system is similar to the plan of that in the higher chordates but lacks a heart.
• The anterior integumentary fold, or oral hood, surrounds a cavity, or vestibule, which leads to the mouth.
• Instead of many gill slits of the amphioxus, there are only seven pairs of gill pouches and slits in the ammocoetes.
• The body consists of a trunk containing immature viscera still in the process of differentiation and a muscular tail for locomotion.
• A small pulsating bulb, functioning as a heart together with the gill bars, aids respiration.
• During the breeding season, egg deposition usually occurs about sunset, and by morning a free-swimming ciliated larva is hatched; this feeds and grows up to 3 months, gradually assuming the adult form, and then takes to burrowing in the sand.
• Larvaceans fail to metamorphose, reaching sexual maturity in the larval state.
• The ammocoete larva of lampreys lives from 2 - 6 years as a burrowing fresh water larva and is a filter feeder with a long slender body, with the front end broader and blunter.
• The nervous system consists a dorsal hollow nerve cord, several ganglia, and nerves.
• The notochord is large and extends from the tip of the tail to a region near the anterior end of the brain.
• At metamorphosis three adhesive papillae with sticky secretions attach the larvae to a permanent substrate.
• Muscular segmentation is also found in the form of myotomes along the dorsal part of the body.