General characters and classifications of protochordata

INTRODUCTION

  • Protochordates are commonly called lower chordates.
  • They lack a head and a cranium, so they are also known as Acraniata.       
  • Protochordates consists of three sub-phyla based on the property of notochord.
  • They are Hemichordata, Urochordata and Cephalochordata.

Hemichordata

  • All hermichordates are marine. Some are solitary and slow moving, others are sedentary and colonial.

Body structure

  • Body is stout and unsegmented, and has a wormlike or vaselike      
  • Three distinct regions namely proboscics, collar and trunk are present.
  • They are bilaterally symmetrical and triploblastic.
  • They have organ-system level of organization.

Appendages

  • There are no locomotory appendages. The collar may bear arms and tentacles.
  • Body wall consists of single layered epidermis and musculature of smooth longitudinal fibres.
  • Their body consists of a true coelom with 3 parts corresponding to the 3 body divisions: an unpaired proboscis coelom, a paired collar coelom and a paired trunk coelom.

Digestive tract and respiration

  • Digestive tract is complete. Proboscis contains a hollow out growth from the gut, called the buccal diverticulum or stomochord and was regarded as notochord in the past.
  • All feed on microorganisms and debris by filtering or ciliary mechanism.
  • Respiration occurs by a pair to numerous pairs of gill slits or through the general body surface.

Circulatory system

  • Circulatory system includes a dorsal heart, two main longitudinal vessels; a dorsal and a ventral, interconnected by small lateral vessels and sinuses.
  • Blood is colorless and without corpuscles.
  • Nervous system and excretory system:
  • Nervous system is diffuse consisting of an epidermal plexus of nerve cells and nerve fibers.
  • Excretory system comprises of a proboscis gland, or glomerulus, situated in the proboscis and connected with the blood vessels.

Reproduction

  • Sexes are separate or united. The gonads may be in several pairs or only one in pair.
  • Fertilization is external or internal. Asexual reproduction may occur.
  • Development may include a free-swimming tornaria larva.
  • Examples:  Balanoglossus (the acorn worm), Cephalodiscus, Rhabdopeura etc.

Urochordata or Tunicata

  • They are marine, mostly sessile, filterfeeders.
  • Body is enclosed in a leathery test or tunic sheath, composed of tunicin (cellulose) so called tunicates.
  • The notochord occurs only in the tail of larva and disappears in the adults called retrogressive metamorphosis.
  • The nerve chord (neural tube) is present in the larva, but is replaced by a single dorsal ganglion in the adult.
  • The gill slits or stigma are numerous, persist in the adults and open into the atrium, instead of opening to the exterior. There are no true gills but are called branchial basket.
  • Circulatory system is of open type. Blood consists of Venadocytes.
  • Excretory system is lacking.
  • Asexual reproduction occurs by budding.

Examples: Herdmania, Salpa, Doliolum etc.

Cephalochordata

  • They are also marine and filter-feeders.
  • The notochord extends up to the cephalic or head region and persists throughout the life.
  •  The nerve chord persists throughout the life, but no brain is formed.
  • Excretion occurs by solenocytes.
  • The gill slits are numerous and persist in the adults. They open in atrium and true gills are absent.
  • The body wall consists of myotomes.
  • Tail persists throughout the life.

Agnatha

  • Eel-like animals with circular mouth, slimy, scale less skin and without jaw and unpaired fins with slender cartilaginous fin rays.
  • The brain is encased in a cartilaginous cranium.
  • Pouched gills supported by cartilaginous branchial basket.
  • Notochord, the chief axial support for the body, persists throughout life and never replaced by vertebrae.
  • Nerves are non-myelinated.
  • Nasal organ impaired, no sympathetic nervous system, no conus arteriosus, no distinct pancreas, no spleen, no genital ducts.

Petromyzontia

  • Comprises widely distributed eel-like, slimy, soft-skinned, scale less agnathans— known variously as lampreys, lamper eels, lamperus, sand pride etc.—belong to a single family Petromyzontidae consisting of eight genera. They are predators on fishes, turtles etc. remaining attached to their body.
  • Most lampreys live in fresh water, but some spend their life in the ocean and return to fresh water only to reproduce.
  • The head and trunk are nearly cylindrical, with the side to side compressed tail region. Mouth is round, encircled by a large basin-like depressed buccal funnel. Inside the buccal funnel are radiating rows of yellow, horny teeth. At the bottom of the buccal funnel, just below the mouth, is a small tongue, bearing horny teeth.
  • A nasohypophysial sac opens by a single nostril on the dorsal surface of the head.
  • In the membranous labyrinth semicircular canals are two.
  • The lateral line sense organs consists of isolated pits opening at the surface.
  • The paired eyes without eyelids are covered by a transparent area of skin.
  • Pharynx ends blindly, and from a branchial basket little behind the eye. On the sides of the basket are seven pairs of small apertures, the gill pouches opening separately. Numerous gill lamellae are present on the inner surfaces of the gill pouches.
  • Paired appendages are absent, but two approximately equal dorsal fins, and a caudal fin present; the second dorsal fin is continuous with the caudal fin.
  • The stomach is absent; a typhlosole present in the intestine.
  • Liver is a large, bilobed organ; gall bladder and bile duct almost entirely absent in adult but present in larval stage.
  • Spleen absent.
  • Pancreas is represented only by cells embedded in the wall of intestine and in liver.
  • Heart two-chambered.
  • The mesonephric kidneys are drained by pronephric ducts which carry only urine.
  • Genital ducts absent and the gametes pass from the gonads to the coelom.
  • Fertilization external, the adults die after spawning.
  • Ammocoete larva is eel-shaped, lacks specialised feeding mechanism of adult and possesses endostyle for food trapping.
  • Rudimentary vertebrae are present in the form of small cartilages on each side of the notochord, two pairs to each metamere.
  • Metameres are ‘W’-shaped.
  • Ninth and tenth cranial nerves not enclosed in cranium.

Examples:   Petromyzon, Lampetra, Entosphenus, Ichthyomyzon, Geotria, Mordacia, etc.

Myxinoidea

  • Exclusively marine, ‘slime eels’ or ‘hag fishes’ descend up to a depth of more than 300 fathoms (540 metres). Some adults are nonparasitic, and most remain attached to muddy bottom where they rest temporarily. They feed on dead or dying animals or prey upon living polychaetes and priapulids.
  • Body cylindrical, 40 to 80 cm long, producing immense amount of slime from the slime glands.
  • Buccal cavity large but the mouth is not expanded into a funnel.
  • Mouth and nostril are surrounded by tentacles.
  • Single median palatine tooth above the oral aperture and two rows of smaller teeth arising from the dental plate on the tongue.
  • The terminal nostril opens into a tubular nasal duct leading to the olfactory organ and is continuous with the nasohypophysial duct opening into the buccal cavity, just anterior to the velum.
  • A narrow caudal fin surrounds the end of tail.
  • External branchial aperture may be one to fourteen pairs. Each branchial aperture communicates by a short efferent duct with one of the gill pouches which is again connected with the pharynx by another tube.
  • There is no trace of neural arch in trunk but in the posterior part of the caudal region both neural canal and notochord are enclosed in a continuous cartilaginous plate. The roof of the’ skull is entirely membranous. Spinal cord is covered with fibrous tissue only.
  • The intestine is wide. Liver consists of separate anterior and posterior portions, the ducts of which open separately in gall bladder. A pancreas-like organ, the insular organ’ present. No definite spleen, but blood forming lymphoid tissue is found scattered in the sub mucosa of gut.
  • Pronephros is retained in adult myxinoids but mesonephros is the functional kidney retaining primitive segmental arrangement.
  • Olfactory lobes large, ventricle reduced and midbrain smaller than that in lamprey.
  • The ventral and dorsal roots of spinal nerves united.
  • The eyes are vestigeal, sunk beneath the skin.
  • Single semicircular canal having an ampulla at each end in the membranous labyrinth.
  • Lateral line sense organ lacking and the pineal organ is absent.
  • Sexes are separate.
  • Larval stage and development unknown.
  • They are the only vertebrates to resemble marine invertebrates in having blood that is isosmotic to sea water.

Examples: Myxine, Paramyxine, Eptatretus (= Bdellostoma), etc.

Gnathostomata

Gnathostomes include all vertebrates with upper and lower jaws. The group embraces a wide range of animals starting from fish at the lowest rung of the evolutionary ladder, the last rung being occupied by anthropoid apes. The jaws were probably derived from the paired anterior gill bars and the gnathostomes evolved from a fish-like ancestor.

Gnathostomes have been divided into seven classes. The primitive Placodermi are extinct. The classes Chondrichthyes, and Osteichthyes are grouped as Pisces or fishes having similar characters due to their primary aquatic adaptation. The last four classes—Amphibia, Reptilia, Aves and Mammalia—are tetrapod’s having common features acquired for terrestrial life.

The paired appendages, the fins supported by dermal fin rays in fishes, an adaptation for aquatic life have been replaced by pentadactyl limbs to carry weight of the body and also to function as locomotory organs in the tetrapod’s. Gills, the aquatic respiratory organs, lost their utility in terrestrial forms and lungs capable of utilizing atmospheric oxygen, evolved.

Shifting from aquatic to terrestrial life exposed the migrants to varied life conditions, the end result of which is the appearance of numerous kinds of animals of’ today, both terrestrial and aquatic, the latter secondarily adopted in their present environment.

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