Obelia Dr Shivani Gupta PGGCG11 Chandigarh Classification Kingdom

Obelia Dr. Shivani Gupta, PGGCG-11, Chandigarh

Classification Kingdom: Animalia Subkingdom: Eumetazoa Phylum: Cnidaria Subphylum: Medusozoa Class: Hydrozoa Subclass: Leptolinae Order: Leptomedusae Suborder: Proboscidoidea Family: Campanulariidae Genus: Obelia

Morphology -The polyps live in colonies of a large numbers of polyps called zooids, which are interconnected and share a continuous gastrovascular cavity. -The zooids are connected to each other and to the substrate by a root-like stolon. Each zooid possesses an oral end called a hydranth, which bears a mouth. -There are usually two types of polyp in the colony. Those specialized for feeding, called gastrozooids, possess a ring of retractable tentacles. -Each tentacle is covered with nematocysts (stinging cells) and once they catch the microscopic prey consisting of protozoa and zooplankton, they withdraw into the hydrotheca , the transparent bell-shaped part of the perisarc, a transparent protective tube. -They then ingest the prey through the single opening into the gastrovascular cavity. -Digestion is extracellular and then becomes intracellular as the food is distributed throughout the colony.

Morphology Obelia Colony

Morphology The phylum Cnidaria is composed of relatively simple animals whose radial symmetrical bodies display definite layers (an outer epidermis or ectoderm, an inner gastrodermis or endoderm, and a poorly developed middle layer, or mesoglea, composed of amoeboid or fibrous cells). -There are no distinct internal organs in this phylum (no head or central nervous system, but there is a nerve net). -The coelenterate body displays some division of labor and cell specialization. -There are epithelio-muscle cells, but no separate muscular system. -There are sensory nerve cells, but no central nervous system. -Gas transport is by simple diffusion, and nutrients are ingested and egested through food vacuoles. =

Polyp The gastrozooids provide food for the entire colony, including the nonfeeding pedicels, stolons, and gonozooids. The periderm of its stem is annulated, or ringed, each ring being an annulus. The polyp is enclosed in a bell-shaped expansion of the perisarc called the hydrotheca (theca = a case). Polyps that are enclosed in this manner are said to be thecate. Obelia, being thecate belongs to the taxon Leptothecatae. Some hydrozoan species (Anthoathecatae) have athecate polyps. -The coenosarc of the gastrozooid is called a hydranth (= water flower). It rests on a transverse shelf of periderm, the diaphragm, at the base of the hydrotheca. The coenosarc of the pedicel passes through a small hole in the center of the diaphragm and is continuous with that of the hydranth. The distal, or free, end of the hydranth bears a single circle of filiform tentacles used to capture prey, sting it, subdue it, and transfer it to the mouth. The tentacles are solid, contractile, and armed with cnidocytes. Cnidocytes are clustered together in batteries that appear as small bumps on the tentacles. -

Polyp In the center of the whorl of tentacles is the erect manubrium, which bears the mouth at its center. -The mouth opens into an expanded region of the coelenteron known as the stomach. -The stomach connects with the canal-like coelenteron that extends throughout the colony. -The gastrodermis of the stomach contains secretory cells that release proteases. These enzymes partially hydrolyze food into small particles and molecules for distribution by ciliary currents to other parts of the colony. -Particles are phagocytized by cells lining the coelenteron and digestion is completed intracellularly. - Nematocysts

Medusa-Polyp

Medusa -The medusa is umbrella-shaped and swims with its convex surface upward so that the tentacles are facing downward into the water. -The tentacles have numerous nematocysts and suctorial pads to assist in capturing prey. Extending into the cavity of the medusa is the manubrium which contains a mouth at its tip surrounded by four oral lobes. -Food organisms enter the mouth and into a gastrovascular cavity. Extending from the manubrium along the inner surfaces are four radial canals that join with the circular canal at the margin and connect with the cavities of the tentacles. These interconnected passageways allow for the distributrion of water, nutrients, and oxygen within the animal as well as waste removal.

Medusa - There are eight spherical statocysts spaced evenly around the margin of the bell. Each is at the base of a tentacle. The statocysts are hollow, thin-walled, epidermal vesicles containing a calcareous statolith and a ciliated sensory epithelium. There are no ocelli in Obelia or other Leptothecatae. -The gonads (testes or ovaries) can also be seen on the undersurface of the medusa. It is dioecious; that is it has the male and female organs in separate organisms. The eggs and sperm are released into the sea where fertilization takes place. The zygote develops into a planula larva which swims about and then settles and is transformed into a polyp.

Life Cycle

Metagenesis -In the metagenesis of some coelenterates, like Aurelia and Obelia, there is alternation between polypoid and medusoid forms. The polyps reproduce asexually originating medusae that then liberate gametes. The male and female gametes unite under water to form the zygote that then develops into larva and later originates another polyp. -The form that reproduces asexually is the polyp. Polyps give birth to medusae by budding in Obelia and by strobilization in Aurelia.