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Hemichordates:

As the name indicates that these are half-chordates

General Characters:

  1. These are marines and deuterostomes.
  2. Their body is divided into three regions proboscis, collar, and trunk
  3. Colome is divided into three cavities, hence called tripartite coeloms
  4. They have ciliated pharyngeal slits
  5. Having an open circulatory system, having no blood vessels
  6. Complete digestive tract
  7. In some cases, the dorsal tubular nerve cord present
Classification:

Hemichordates are classified into 2 general classes

(1)

ENTEROPNEUSTA

(2)

PTEROBRONCHIA

   1.    ENTEROPNEUSTA

They are also called ACORN WORM or BALANGLOSSES

Enteropneusta is a Greek term Entero term used for intestine, while The term Pneusta means Breathing

General Characters:

i.    They are marine

ii.    Size range from 10-40 cm in length.

iii.    Having 100 Species

iv.    Mostly occupies U-shaped burrows

v.    They are called ACORN WORM because their proboscis are exposed outside and also they look like ACORN FRUITE

vi.    Having tripartite coelome

vii.    Ciliated epidermis and glands covered the acorn worm

viii.    The mouth located with the Projunction between the collar and proboscis

ix.    Pharyngeal slits are present in the position laterally on the trunk

x.    Small diverticulum of the gut tract called the BUBBLE DIVERTICULUM extends into the proboscis.

xi.    Buccal diverticulum help indigestion.

Figure: Phyrangeal Slits

Figure: Class Enteropeusta Longitudinal Section showing the Proboscis

Maintenance Functions:

i.    Nutrition and digestive system of acorn worm

  • Cilia and mucus help the acorn worm during feeding cilia help
  •  in the movement of food from the mouth to the diverticulum,
  •  while mucus moist the food which helps indigestion.
  • The digestive system is a simple tube form
  • Food digested in diverticule of the gut called HEPATITIC
  •  SACS, releases enzymes.
  • Figure: Digestive System
  • Water current brings their food.

ii.    Nervous System:

The nervous system consists of sensory receptors that are present throughout the entire body.

      iii.    Respiratory System:

Respiration occurs in them in two ways.

  • Diffusion occurs through the general body surface.
  • Respiration also occurs through pharyngeal slits.
    iv.    Circulatory system:

  • Having an open circulatory system
Diagram of Circulatory System

    v.    Dorsal Contractile vessel:

  • It brings the blood towards the anterior.
  • Ventral contractile vissel:
  • Bring the blood towards the posterior end of the body, it filters the blood and releases waste into the coelom of the proboscis
  • Glomerulus:
  • Which filters the blood and releases the wastes in the coelom of a proboscis.
  • The blood of acorn worms is colorless, lacks cellular elements, and distributes nutrients and wastes.

vi.    Reproduction & Development:

  • Enteropneusta are dioecious having sexes are separate
  • Two rows of the gonads are present on the anterior region of the trunk, each row opens separately to the outside.
  • Fertilization is external
  • They show swamping behaviors.
  • Figure: Reproductive System

  • Spawning by one worm.

Figure: Tornaria Larvae
Ciliated larvae called tornaria swim in the plankton for several days to a few weeks. The larvae settled to the substrate and gradually transform into adults.

2.    Class Pterobronchia:

Ptero means wings feathers and bronchia means gills.

  1. General Characters:
  2. It is a small class of hemichordates found mostly in deep ocean water.
  3. Having 30 species
  4. They are very small ranging, from 0.1-5mm.
  5. Their body is divided into three regions.
  6. Having single pharyngeal slits.
  7. Having a trunk containing U-Shape Gut.
    General Structure of Member of Class Ptero Bronchia

Mountainous Functions:

i.    Nutrition:

  • Basically, filter feeders are on the top of their mouth, there are tentacles, on the top of tentacles there are cilia. They push water current to the mouthy selecting food.

ii.    Respiration and Excretion:

  • They are so small that respiration takes place on the general body surface.
  • Generally, respiration and excretion occur by diffusion.

iii.    Reproduction:

  • Reproduce asexually
  • Budding is common in pterobrunchia which is responsible for colony formation
  • They possess ½ gonads in the anterior portion of the trunk.
  • They are mostly delicious
  • External fertilization results in the development of planula-like larvae, that live for a time in the tubes of females, these none functioning larvae eventually leave the female's tube and settled to the substrate for a cocoon and metamorphosis occurs to form adults.

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