Herdmania- Sea Squirts

Systematic position:

Habit and Habitat:

Herdmania is a sedentary filter-feeding tunicate found in marine habitats. The animal often occurs in a gregarious form. These are marine animals, found in all seas at all depths.

  • Herdmania Momus is one of the most commonly encountered species of ascidians. It is sometimes referred as red-throated ascidian.
  • Each animal is found attached to substratum usually separately, at its posteroventral end by means of the foot.
  • A large number of organisms inhabit the test of Herdmania, some of these are merely attached to its surface, while others are more or less embedded within its substance.
  • Sometimes these are found attached to a living gastropod shell showing commensalism. e.g. T. Trapa, Xancus, Turbinella pyrum, etc. Herdmania protects the gastropod from enemies as it is unpalatable due to its spicules, and gastropod moves it from one place to another, by this they get good food, and oxygen, etc.

Body Wall:

The body wall of Herdmania is called the mantle. It is thick and muscular in the anterodorsal region of the body and thin, non-muscular, transparent in the posteroventral region. It shows the epidermis, epidermis, mesenchyme, and inner epidermis.

  • The body wall protects visceral organs, the outer epidermis secretes the test, while the musculature brings contraction of the body and the siphons
  • Absence of segmentation.

herdmania

Digestive system:

In Herdmania the digestive system is coiled and complete. The digestive system containing the following parts:

Mouth: It forms the opening of branchial siphon marking the anterior end of the body. It is guarded by four lobes derived from the test.

Buccal Cavity: Mouth leads into a laterally compressed cavity of branchial siphon, called buccal cavity or stomodaeum

Pharynx: Buccal cavity leads into the pharynx which occupies the major part of the body or atrial cavity. It is differentiated into a prebranchial zone (smaller anterior region having smooth walls without folds, cilia, and stigmata or gill slits) and a branchial sac (the larger posterior region of the pharynx and is also known as the branchial basket).

Other structures associated with the Branchial sac are:

  • Trabeculae – Trabeculae are hollow strands, containing a blood vessel, connecting the outer wall of the branchial sac to the mantle
  • Dorsal Lamina- It is a thin flap or folds extended mid-dorsally from the posterior pharyngeal band
  • Endostyle- This is a shallow mid-ventral groove lying on the floor of the branchial sac.
  • Oesophageal area: The posterior-most region of the branchial sac has a small circular oesophageal area. Its opening is guarded by two semi-circular lips. This area is devoid of blood vessels, folds, and stigmata
  • Esophagus: It is a very short, curved, and thick-walled tube. It contains four longitudinal ciliated grooves, connects the branchial sac with the stomach.
  • Stomach: It is wider than the oesophagus, thin-walled, the sphincter at both ends, and has a smooth inner lining
  • Intestine: It is a thin-walled, U-shaped tube formed by a proximal, ventral, or descending limb and a distal dorsal or ascending limb, both united anteriorly. The intestinal loop thus formed encloses the left gonad.
  • Rectum: Intestine leads into the rectum, internally lined by cilia. It curves dorsally to open into the atrium or cloaca through the anus. The anus is guarded by four lips.
  • Cloaca: The atrium or cloaca leads into the atrial siphon and opens outside through the atrial aperture

Herdmania- Sea Squirts Internal structure

Nervous system:

The hollow dorsal nerve cord of all ascidians is lost during larval metamorphosis leaving the adult body plan, divergent from other chordates.

  • The neural gland lies beneath the brain but actually has no nerves.
  • Sensory organs are also lost during the transition from larvae to adult but sensory cells remain on the surface of the siphons, Buccal tentacles on the atrium are used to control the water flow through the pharynx

Circulatory system:

The circulatory system is well developed and closed type. It includes –

  • Heart and pericardium– is a non-contractile, elongated and transparent tube about 7.0 cm long and 3.0 mm wide running obliquely below the right gonad. In Herdmania, there is no valve in the heart and the flow of blood is maintained by peristaltic waves.
    • It is closed at both ends and is filled with a colourless pericardial fluid with corpuscles similar to those of blood.
    • The wall of the heart is thick, made of connective tissue and contains blood sinuses, and is internally lined by squamous epithelium.
    • The heart is enclosed within the pericardium and is attached to its wall along its entire length by a thin mesentery-like connective tissue flap.
    • Both ends of the heart are open, with no valve but a pear-shaped body present midway in the pericardial body which probably regulates the flow of blood in the heart.
  • Blood vessels– The major blood vessels have definite walls, but the smaller ones lack them.
  • Blood– Blood is slightly reddish, transparent, and hypertonic to seawater. It contains a few colourless, amoeboid leucocytes, 6 to 7 types of coloured corpuscles with or without a nucleus, and the nephrocytes having vacuoles and colloidal cytoplasm.
    • Orange, Yellowish-brown pigments may be found in corpuscles, but never of red colour and don’t take part in respiration.

Reproductive system:

  • Herdmania is hermaphrodite and oviparous.
  • It consists a single long gonad.
  • The organism has a long deep orange ovary, overlain by a light orange test
  • The oviduct and sperm ducts are separate, run parallel to the intestine, and open into the atrium near the anus.
  • Eggs and sperms travel to the cloaca in small amounts nightly, through the process called trickle spawning.
  • The fertilization takes place in the sea.
  • After fertilization, zygote formation and cleavage of larva known as tadpole larva (lead a short free-swimming life) hatch out of the egg, it becomes an adult after retrogressive metamorphosis.

gonads of herdmania

Retrogressive metamorphosis in Herdmania:

The term metamorphosis means a change from juvenile to adult stage in which the larval stage is quite different from the adult stage.

  • In retrogressive metamorphosis, the larva possesses advanced characters which are lost during the development and the adult is either sedentary or degenerated with primitive characters.
  • Urochordate adults, being sedentary show degenerative characters while the free-swimming tadpole larva shows advanced chordate characters lost during metamorphosis.
  • Both tail and notochord are gradually absorbed in the body during metamorphosis.
  • The hollow nerve chord is reduced into solid nerve ganglion on the dorsal side.
  • The sensory organs named ocellus and statocyst are lost during the transition from larvae to adult.
  • After metamorphosis, Herdmania is transformed into a bag-like sedentary animal attached to the rock by a foot and having branchial and atrial openings for inlet and outlet of water respectively.
  • The pharynx enlarges with a large number of stigma for filter feeding.
  • Digestive system is well developed.

Affinities with Chordates:

  • Presence of dorsal tubular nerve cord.
  • Presence of notochord.
  • Well-developed pharynx with gill slits.
  • Presence of endostyle on the ventral side of the pharynx
  • Presence of atrium around the pharynx
  • Presence of post-anal tail with the tail fin

Affinities with Amphioxus (Cephalochordates):

  • Presence of Notochord
  • Presence of dorsal tubular nerve cord
  • Presence of large Pharynx with gill slits.
  • Presence of muscle band
  • Presence of atrium and atriopore

These animals reoffered as primitive and ancestral forms of Chordates as a whole.

 

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