Herdmanla -External characters
Why the Hermania is considered as degenerated chordata?
a)Due to absence of kidney
b)Due to absence of head
c)Due to absence of brain box
d)Due to all above character
The urochordates are exclucively marine animals living on the substratum. The most familiar member of the group is Hardmania; it possesses two siphons for the entry and exit of water. Characteristically they have a larval stage which undergoes retrogressive metamorphosis giving rise to the adult animal.Urochordates show all the basic characters of chordata at one or the other stage of life, they are supposed to be lower than Cephalochordata but higher than hemichorates from evolutionary point of view.
TADPOLE LARVA The tadpole larva of Herdmania is only 1-2 mm long when it hatches out of the egg. It does not feed and hence has only 3 hours of survival during which it has to swim about in search of a suitable substratum for attachment. The larva needs advanced features for its free swimming existence, which is so necessary for dispersal of the population to distant places which the sedentary adult has no means to do. The advanced chordate characters of the larva:
- There is a rod-like notochord in the tail to which are attached muscle bands for swimming.
- There is a dorsal hollow nerve cord which is enlarged to form brain at the anterior end. A photoreceptor ocellus and a balancing organ, the statocyst are attached to the brain.
- There are only two pairs of gill slits in pharynx but the mouth is closed by a membrane and intestine is rudimentary
External characters of Herdmania
Herdmania is a simple ascidian, In Indian ocean this genus Is represented by 4 species. ,herdmania
1. Herdmania pallida, 2. H.ennurensis, 3. H.mauritiana 4. H. ceylonica.
Herdmania belongs to,
Herdmania is a marine and sedentary animal. It is fixed to rocky substratum by a flat base. When it is disturbed, it suddenly contracts its body, and emits inner contents with force through its apertures. Hence it is called Sea squirt.
External Features :-
It is potato like in shape. It is pink in colour. On the free side, body shows two projections, the brançhial and atrial siphons. The branchial siphon is short. The branchial siphon shows a branchial aperture or the mouth. The atrial siphon is longer. It bears the atrial aperture. Both the openings are bounded by four lips.
Herdmania - external characters.herdmania
Test of Herdmania:The body of this animal is covered and protected by test. It Ls a thick, leathery covering of the body.It is secreted by the epidermis of the body wall. It has matrix, corpuscles, fibrils, blood vessels and spicules.
The matrix is composed of tunicin, which is cellulose. The cells in the test are of six types, large eosinophilous cells, amoeboid cells, small eosinophilous cells, vacuolated cells, receptor cells and nerve cells.
Fine fibrils present in the matrix. In the test blood vessels are present. In the test the spicules are calcareous spicules. They are microscleres, and megascleres.Herdmania-test
The test protects the body. Anchors the animals to substratum. Its spicules form a supporting frame work.
Herdmania Body wall :
The body-wall of Herdmania is called Mantle. It is thick, and muscular in the antero-dorsal region of the body. It is thin, non-muscular and transparent in the postero ventral region. It shows epidermis, mesenchyme, and inner epidermis.
1.Epidermis : It Is single layer of cells. It covers the bronchial and atrial apertures and siphons. The epidermis is interrupted at places where spicules and blood-vessels pass from the mantle into the test.
. 2. Mesènchyme: It develops from the mesoderm. It has connective tissue containing blood-sinuses, muscle-fibers, nerve fibers and cells. The muscle fibers are long and flat. They contain large nuclei.Herdmania Body wall
3. Inner Epidermis: It is single layer of flat cells. It forms the lining of the atrial cavity. .
‘1.The body-wall protects visceral organs.
2. The outer epidermis secretes the test.
3. The musculature brings contraction of the body and the siphons.
In Herdmania coelome is not developed. Atrium is a spacious ectoderm lined cavity. it is covered by the mantle A part of the atrium surrounds the pharynx. The stigmata of the pharynx open into this cavity. Part of the atrium is dorsal to the pharynx. It is very wide and is called cloaca. The rectum and gonoducts open into this. The cloaca opens to the exterior through atrial siphon and trial aperture. The atrial siphon shows a ring of processes called atrial tentacles at its base.
next topic is Herdmania reproduction.
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