Retrogressive Metamorphosis is a peculiar types of metamorphosis found in herdmania and other urochordates. During development, certain characters of the advanced grade of organization of the larva completely disappear in the adult. It is probably due to the sedentary mode of life of the adult.
Retrogressive metamorphosis in Herdmania
Retrogressive Metamorphosis in herdmania or Ascidian follows disappearing of certain advanced characters of well developed larva towards the simplification of its organization. An adult herdmania or ascidian appears as a sea potato or a bag. The body is laterally compressed and somewhat oblong or rectangular in shape as seen herdmania diagram.
In herdmania, retrogressive metamorphosis is divided by the following stages;
- Pre-larval Stage
- Larval Stage
- Post Larval Stage
Pre-larval stage of Retrogressive Metamorphosis
In herdmania two large gonad are found that is embedded in teh mantle and bulging into peribranchial or atrial cavity. The right gonad is lies just parallel and dorsal to pericardium. While the left gonad situated within the intestinal loop.
Structure of each gonad:
Each gonad is measured about 3 to 4 centimetre in length about 1.0 centimetre in width and about 2 to 3 millimetre in thickness and include about 10 to 25 almost the rounded lobes are arrenged in two rows one on either side of a central axis.
Each adult gonidial lobe is hermaphrodite, elongated and lobulated bearing an outer , brick red, cortical, outer testicular zone with rounded elevations and a small pinkish inner ovarian zone.
The testicular zone is composed of small hollow and elongated spermatic caecae or tubules which is linned by germinal epithelium, forming spermatogonia and spermatozoa. The ovarian zone has a a lobulated surface due to formation of ovarian follicles witch contain rounded ova or egg on various stages of development.
In herdmania each gonads has two gonoducts, known as spermatic duct and oviduct, which is runing along the central axis. The oviduct is wider in size formed by the union of ovarian ductules each from ovarian zone of each lobe. It opens into dorsla atrial cavity or cloaca, after short distance of anus, by an oviducal aperture coverd by four thick lips.
The Spermatic duct is very narrower, It is formed by the union of spermatic ductules each from testicular zone of each lobe. It runs along the inner side of the oviduct and indipendentally opens into cloaca by spermiducal aperture a just short behind the oviducal aperture. The oviduct and spermatic duct both are internally lined by many hair like stucture called cilia.
In herdmaina the gametes are produced by the testicular lobes and ovarian lobes. The testicular lobes produce sperm or spermatozoa and the ovarian lobes produce egg or ovum.
The sperm is a reproductive cell produced by testicular lobes and a single sperm consist of large head, middle pice or nack and vary long straight locomotory tail. The size of mature sperm is about 4 micra in length. Each sperm has broad and nucleated head on which a beak-like acrosome found. The sperm are polymorphic so there are three types of acrosome shorter, equal or longer than head.
The ovum is a reproductive cell produced by the ovarian lobes. A mature ovum measures about 0.3 mm in diameter, the yellowish granulated yolk in distributed in cytoplasm. In ovum, the large nucleus situated in one side, which contain conspicuous eccentric nucleolus and also a dense layer of chromatin granules which lies beneath the nuclear membrane. each ovum is surrounded three membranes which are given bellow;
- Vitelline membarane
- Inner chorion
- Outer Chorion
The vitelline membrane is a thin secreted by the ovum itself and the follecular cells of the ovary secrete two chorion membranes. Outer chorion ovum present juct above the inner chorion and the inner chorian present just below the outer chorion.
In herdmania the crossed and external fertilization is occur, although herdmania is hermaphrodite, self fertilization is not occcur due to protohyny. The female gamete mature earlier than sperm and discharged into sea water after it the mature male gamete fused with a ovum in the sea water.
The zyogote undergoes holoblastic, somewhat unequal and determinate zygote, deeloped into coeloblastula containing blastocoel cavity filled with fluid. Blastula develops into gastrula by the process of invegiantion consisting of archenteron cavity which opens outside through blastopore. The gastrualla developes into larva which is claled ascidian tadpole larva.
The cleavage is holoblastic and starts after about half an hour of fertilization. The holoblastic cleavage occurs in the zygote and it becomes hollow ball called blastula. The blastula contains blastocoel, upper layered cells called mircromares and lower cells known as a megameres. The former layer forms ectoderm while the another forms endoderm.
The blastula froms gastula by epiboly. The division of micromeres causes invagination in the blastocoel and thus archenteron in formed which opens in blastopore. The blastopore becomes closed for the following further development. The gastula increase on longitudinal axis, its dorsal surfure becomes flattened and ventral surface becomes cortex. Finally , after subsequent changes gastula develops into larva which is called Ascidian Tadpole Larva.
Larval Stage of Retrogressive Metamorphosis
Structure of an ascidian tadpole larva shown in the figure;
The tadpole is an elongated somewhat cylindrical in shape and measures about 1.5 millimetres in length and 0.3 millimetres in width. It is surrounded by a thin and transparent covering, the tast which becomes flattened in the region and forms a tail fin.
- Dorsal to notochord is present in nerve cord which is in the interior part swells up to form a nerve ganglion or sensory vesicle.
- The sensory or cerebral vesicle is hollow and oval, in which lodged to two ocelli and otocyst.
- The ocelli lie in the anteroventral and posterodorsal position.
- The alimentary Canal is fully formed.
- The mouth is situated at the anterodorsal end of the larva leading into a perforated sac-like pharynx with a ventral endostyle.
- The pharynx is followed by oesophagus, stomach and intestine.
- The anus is situated on the left side of the atrium.
- The ventrally situated heart lies in between the pharynx and stomach.
- All Around The alimentary canal is a cavity, the atrial cavity, which communicates with the exterior through atriopore.
Post larval stage of Retrogressive Metamorphosis
Retrogressive metamorphosis changes:
- First of all, the tail starts disappearing the disappearance of the tail is affected by resorption.
- The epidermal ampullae of act as an organ of attachment as well as respiration.
- The adhesive papillae are completely lost.
- The visceral ganglion is converted into a visceral nerve.
Progressive metamorphosis changes:
- During metamorphosis besides retrogressive changes, certain progressive changes also take place.
- The neural gland is formed by the ciliated diverticula which have its origin from the anterior part of the pharynx.
- The pharynx enlarges and the number of gill slits is also increased. The stomach and intestine also grow in size.
- The gonads oviduct and vas deferens are start developing from mesoderm.
- The thick covering of test develops around the animal.
- In progressive metamorphosis changes the liver is formed.
Conclusion of retrogressive metamorphosis:
As a result of retrogressive metamorphosis free-swimming photo-positive and geo-negative tadpole ascidian, which is more advanced in the grade of organisation, change into a fixed, inactive geo-positive and photo-negative simple adult. The chordate characters, notochord and, nerve cord and, spinal sense organs present in Lava as completely lost in adult.
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