BSc 1st Year Lower Non-chordates Obelia Sample Model Practice Question Answer Papers
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Index for BSc 1st Year Lower Non-chordates Obelia
Morphology and Histology of obelia: Page 1
The division of Labour: Page 2
Radial symmetry and Development of Obelia: Page 3
The structure and Life History of Obelia: Page 4
Homologies with the Polyp: Page 5
BSc 1st Year Lower Non-chordates Obelia Question Answers
Q. 1. Describe various types of zooids found in Obelia colony.
Give a detailed account of morphology and histology of mediusal of obelia.
What is polymorphism? Explain it with reference to various types of zooids found in Obelia colony.
Polymorphism (Poly – many, morph-form) represents a phenomenon in which individuals of a colony assume different forms to carry out different functions. The individuals of the colony are called zooids. Such a colony is called polymorphic and the phenomenon is known as polymorphism.
Obelia colony exhibits division of labour and polymorphism. Its colony bears two types of zooids: Gastrozooids or hydranths and gonozooids or blastostyles The mature blastostyles produce sexual individuals, the medusa. Medusae are sexual zooids.
Due to the presence of three types of zooids. Obelia colony is said to be trimorphic.
A. Hydroid Colony
1. Morphology and Structure of Colony
1. External features: The hydroid colony of Obelia is delicate, semitransparent, and whitish or light brown in colour. Each hydroid is a colony of different types of individuals, called zooids. A hydroid colony consists of a vertical branching stem, called hydrocaulus 2-3 cm above the root-like stolon or hydrorhiza.
Each hydrocaulus gives off short, lateral branches in an alternate manner. The lateral branches bear a zooid, an individual of the colony. These are nutritive zooids, the gastrozooids or polyps or hydranths. In the axil of olden polyps develop cylindrical reproductive zooids, the gonozooids or gonangia or blastostyles. Thus Obelia Colony is dimorphic. When blastostyles develop saucer-shaped bodies, the medusae, colony becomes trimorphic.
2. Coenosarc: Branches and zooids or the colony consist of an inner tubular and living portion, the coenosarc. It encloses a canal called the coenosarcal or gastrovascular cavity. It is continuous with the canal of zooids. Its wall consists of an outer epidermis and an inner gastrodermis, with a gelatinous mesogloea in between.
3. Perisarc: Coenosarc is surrounded on all sides by a yellow-brown, tough, transparent, and nonliving chitinous layer. It is called perisarc and secreted by the epidermis of coenosarc. It serves as a protective supporting exoskeleton. In the beginning, perisarc is in close contact with coenosarc but in an old colony, it becomes separated by a space. At certain places, epidermal cells of the coenosarc remain in contact with the perisarc. At the base of each zooid and also below the base of lateral branches, perisarc ring-like constrictions, the perisacral annuli.
Perisarc provides rigidity to the colony and annular constrictions all bending movements under the influence of water current
4. Zooids: These represent the individual forms of a coelenterate colony. The hydroid colony of Obelia bears two types of individuals or zooids.
1. Polyps or hydranths or gastrozooids and
2. Gonangia or gonozooids or blastostyles.
1. Polyp or Hydranth or Gastrozooid
Position: Hydranths of Obelia are borne on the distal ends of the hydrocauli or their branches.
Shape: Each hydranth is a conical, vase-like sac opening at both ends. It is yellowish in colour and is radially symmetrical.
Structure: The body of hydranth can be distinguished into the following parts:
1. Stalk: The basal part of the hydranth is a hollow stalk by which it is attached to the hydrocaulus.
2. Hypostome: The distal 1/3rd part of zooids is a conical projection and is known as hypostome or manubrium. It is bears terminal mouth, surrounded by a ring of 24 tentacles. The tentacles are solid and filiform. The mouth opens in the gastrovascular cavity of polyp. It is connected with coenosarcal canal of hydrocaulus.
Histology: The body wall of hydranth consists of two layers:
1. the outer layer of the epidermis, and
2. the inner layer of gastrodermis.
The two layers are continuous with one another at the mouth and the two are joined together by a non-cellular, gelatinous mesogloea. The polyp is protected externally by a thin covering of perisarc, which expands into a transparent conical cup and forms the hydrotheca. At the base of hydranth, the hydrotheca forms a horizontal shelf, a resting platform for the polyp. It checks the retraction of polyp into coenosarc.
Function: The polyps or hydranths are the nutritive zooids of the colony. These are carnivorous and feed upon small aquatic animals.
2. Blastostyles or Gonangia
Structure: When Obelia colony attains its full development, it produces special zooids called blastostyles. These arise as buds in the axils of older hydranths. The blastostyles are modified for asexual reproduction. These are situated in the axil of polyps, and occur towards the proximal region. Each blastostyle is narrow, elongated, and tubular having a two-layered body wall, which encloses the gastrovascular cavity. It is rudimentary and does not open to the exterior. The distal free end is closed by a flattened disc. The mouth and tentacles are absent, hence it cannot feed. Each blastostyle is surrounded by a loose, glassy, transparent perisarc, known as gonotheca.
Function: The blastostyles are modified zooids meant for asexual reproduction. These produce saucer-shaped or bell-shaped bodies arranged along a central rod in different stages of development. These are known as medusa buds. These develop in the form of minute buds from the wall of blastostyle. On attaining maturity the medusae detach and start an independent life.
The medusae are the sexual zooids of the Obelia colony. These are produced as hollow buds from the coenosarc of the blastostyle during spring and summer.
Morphology and Structure of Medusa
1. Shape and size: The fully grown medusa of Obelia is a radially symmetrical, saucer-shaped, or bell-shaped structure with the outer cortex and inner concave surface. The former is known as the exumbrella surface and the latter as the subumbrella surface.
2. Manubrium : From the centre of the concave subumbrella surface arises a short, hollow manubrium. At its free end, it bears a terminal four-sided mouth.
3. Velum: The free circular edge of the umbrella is produced inwards in a rudimentary fold called velum.
4. Tentacles: Rim of umbrella bears numerous short tentacles. These are highly contractile. In a young medusa, there are 16 tentacles, the number of which gradually increases with age. These sixteen tentacles are arranged in definite groups. There are four per-radial tentacles situated against the four radial canals, four interradial tentacles present on radii which bisect the per-radii and eight adradial tentacles found in between the per-radial and inter-radial tentacles. The base of the adradial tentacles are thickened into fluid-filled sacs, the statocysts. These are marginal sense organs and inside their cavities are found particles of CaCO3 lithocysts or statoliths. The bases of all sixteen tentacles are swollen into bulbs due to the accumulation of interstitial cells.
5. Gastrovascular cavity: The rectangular mouth leads into the stomach or gastrovascular cavity. Stomach leads into four radial canals. The radial canals open into the circular canal near the margin of medusa.
Histology of medusa: Medusa is disploblastic having an outer layer of ectoderm and the inner layer of endoderm, cemented together by non-cellular mesogloea. Both the surfaces of the umbrella are surrounded by ectoderm while the gastrodermis is limited to the gastric cavity, radial canals, and the circular canal. It forms endodermal lamella between the radial canals and exumbrellar and subumbrellar layers of ectoderm. The velum is the double fold of ectoderm. Mesogloea lies everywhere in between the ectoderm and endoderm.
6. Nervous system: In the medusa, nerve cells and nerve fibres join to form two circular nerve rings, one on each side of mesogloea in the margin of medusa. The upper or inner nerve ring supplies the tentacles and the lower or outer nerve ring supplies the musculature and statocysts.
7. Statocysts: There are eight marginal receptor organs or statocysts situated at the bases of eight adradial tentacles on the subumbrella surface, just inside the bell margin. Each statocyst consists of a minute, fluid-filled ectodermal sac. It contains a movable particle of CaCO3 called statolith or otolith. The wall of statocyst is made of epithelial cells.
Function: Statocysts are the organs of equilibrium and muscular nation. They are associated with the active free-swimming habit of medusa.
8. Reproductive organs: Medusae are sexual reproductive zooids or gonozooids possessing gonads. These are dioecious i.e. testes and ovaries are borne by separate medusae. Each medusa bears only four gonads in the middle of each radial canal. These are formed as ventral diverticulae of radial canals. These mature on the escape of medusae from gonangia. Each gonad is ovoid and knob-like with an outer covering of epidermis and an inner lining of gastrodermis. The space between these two layers is filled with a mass of germ cells or sex cells. These produce sperms or eggs.