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Academic Script
AIM- “To Study the Specimens of Hydra, Obelia, Aurelia, Seaanemone and Gorgonia”
INTRODUCTION:
All the above mentioned specimens follow under Phylum
Coelenterata. The Coelenterates are colourful group of water dwelling, radially
symmetrical and lower invertebrate animals including a large number of familiar and
beautiful forms such as Hydra, Obelia, Aurelia, Sea-anemone, Gorgonia, etc.
HYDRA
Classification:
Phylum:
Coelenterata
 Tissue grade of organization, diploblastic and acoelomate.
Class:
Hydrozoa
 Hydroids containing medusa with velum.
Order:
Hydroidea
 Polypoid generation is well developed.
Sub-order: Athecata
 Hydrotheca is absent.
Genus:
Hydra
Habit and Habitat: Hydra is a solitary and sedentary fresh-water form, found
attached to stones, rocks and weeds.
Distribution: It is cosmopolitan, but most common in India, Canada and U.S.A.
Comments: Cylindrical body is tube-like measuring approximately 1.3 cm in length.
Anterior end contains mouth or hypostome, which is crowned with 6 to 10 filiform
nematocyst bearing tentacles. Proximal end of the body contains pedal or basal disc
or foot meant for attachment to the substratum. Body is diploblastic, consisting of
outer ectoderm and inner endoderm. Body wall encloses a gastrovascular cavity
extending into the tentacles. In female Hydra, ovary and buds are found on sides in
mid position. In male Hydra 3 or 4 pairs of testes, are found anteriorly and a full grown
bud with tentacles on side. Testes develop near the oral end and ovaries near the base. It
reproduces sexually by fusion of sperms and ova. During asexual reproduction, lateral
buds develop on the sides of the body which later on detach and develop into new
Hydra.
OBELIA
Classification
Phylum: Cnidaria
 Tissue grade of organization, diploblastic and acoelomate.
 This is sub-phylum of coelentereta
Class: Hydrozoa
Order: Hydroidea
Sub-order: Leptomedusae
 Polyps athecate, i.e., not covered by hydrotheca.
 Medusa with gonads on manubrium.
Genus: Obelia
Habit and habitat: The hydroid phase of Obelia is a typical marine, small,
branching, sedentary, and colonial form. It lives at 80 meters depth.
Distribution: Obelia has a worldwide distribution except the high-arctic and
Antarctic seas.
Comments: The colony of Obelia looks like a delicate, mossy or fur-like growth,
whitish or light brown. It consist horizontal and vertical filaments. The basal or
horizontal anchoring portion of a colony is like a creeping root or complicated
meshwork, encrusting over the surface of the substratum. It is called the hydrorhiza or
rhizome (Gr., rhizo, root). The hydrorhiza gives off several slender, upright or vertical
threads, called hydrocauli. Each hydrocaulus forms the main stem or axis of a colony
and measures about 2 to 3 cm in height. The stem and the zooids of the colony are made
up of a living hollow, cellular tube the coenosarc. Through its life cycle, Obelia take
two forms: polyp and medusa. They are diploblastic, with two true tissue layers –
an epidermis (ectodermis) and a gastrodermis (endodermis), with a jellylike mesoglea filling the area between the two true tissue layers. A gastrovascular
cavity is present where the digestion starts and later becomes intracellular. They have
incomplete digestive tracts where the food enters, is digested, and expelled through the
same opening. During the polyp stage, the mouth is situated at the top of the body,
surrounded by tentacles, whereas during the medusa stage, the mouth is situated at the
distal end of the main body structure. Four gonads lie in this main body structure,
or manubrium. When food is taken in through the mouth, it enters the manubrium. The
food is then distributed through a canal system, consisting of four radial canals and an
outer ring. Defense and the capture of prey are helped by unique stinging cells
called cnidocytes that contain nematocysts, which are triggered by the cnidocil.
AURELIA
Classification:
Phylum:
Coelenterata
Class:
Scyphozoa
 Medusa well developed, hydroid reduced.
Order:
Semaeostomeae
 Free-swimming scyphomedusae with umbrella-like disk and oral lobes.
Family:
Ulmariidae
 Simple or branched radial canals and a ring canal.
Genus:
Aurelia (Jelly-fish)
Habit and habitat: Aurelia is a solitary marine and medusoid jelly-fish often living
in brackish water. It inhabits coastal water of all oceans in all zones and in abundant
number, mostly living in warm and temperate latitudes.
Distribution: Very common along the entire Atlantic coast and Pacific coast and only
form reported in temperate regions.
Comments:
Commonly called as Jelly-fish measuring approximately 30 cm in
diameter. Body is gelatinous, transparent, bluish, white, reddish or pinkish. Saucershaped body is distinguished into convex ex-umbrellar and concave sub-umbrellar
surfaces. Various other structures seen are gastric filaments, sub-genital pits and
velarium. Sub-umbrellar margin contains marginal tentacles having stinging cells and
8 marginal lappets having sense organs or tentaculocysts in 8 lobes. Mouth 4cornered. Each corner is drawn out into an oral arm along perradii. Inter-radial,
adradial and per-radial gastrovascular canals open in circular canal. Jelly-fish
feeds on molluscs, crustaceans, protozoans, nematodes, rotifers, diatoms, polychaetes
and copepods. It swims by rhythmic contractions of umbrellar surfaces. Males and
females are separate individuals. Life-cycle shows alteration of generation.
In the life cycle of Aurelia, elongated larva metamorphose into a small trumpt-shaped or
hydra like polyp, about 5mm high and without perisarc known as Scyphistoma. It
represents the hydroid or polyp phase in the life cycle of Aurelia. Its proximal part is
narrowed into a stalk like organ attached to the substratum by a basal disc. Tentacle
buds are found at the region of mouth. In Aurelia, they are represented in larval stage
only. The Scyphistoma feeds and grows upto 12mm and may survive several months in
this stage.
SEA ANEMONE
Classification:
Phylum:
Coelenterata
Class:
Anthozoa, Actinozoa
 Solitary or colonial, exclusively marine.
Order:
Actiniaria
 Solitary or colonial anemone-like polyps with one siphonoglyph and
no skeleton.
 Tentacles and septa paired and often in multiples of six.
 Examples: Fungia, Acropora, Oculina.
Habit and habitat: The sea anemones live mostly attached to rocks, shells, seaweeds, submerged timbers and other strata from low tide marks to depths of 6,000.
Distribution: It ranges to the Pacific and the North Atlantic coasts.
External features: Brightly coloured mostly white, green, blue, orange, red or a
combination of many colours but are usually brownish or yellowish. It has diameter of
about one meter. Body is short, cylindrical, radially symmetrical and generally longer
than broad when fully extended. Body is divided in to three regions- pedal disc, column
and oral disc. The oral end of the body is flat or slightly convex, short and delicate. It
bears numerous short hollow marginal tentacles. Many Sea anemones form symbiotic
relationships with single-celled algae, whether zooxanthellae or zoochlorellae, that live
within their cells. The sea anemone is considered to be the flower of the sea. Sea
anemones look like plants, but they are really meat-eating animals. Sea anemones come
in different sizes and many different colors. They spend most of their lives at one place.
They can attach themselves to rocks. Some even bury themselves in the mud.
GORGONIA
Classification:
Phylum:
Coelenterata
Class:
Anthozoa
Sub-class: Octocorallia
 Polyps with 8 tantacles, bioluminescent.
Order:
Gorgonaceae
 Horny corals, polyps short not reaching upto base.
Sub-order: Scleraxonia
 Axial spicule is present.
Family:
Gorgonida
 Slender stem, thin, coenenchyme and partially retractile anthocodia.
Genus:
Gorgonia (Sea fan)
Habit and habitat: Gorgonia is a colonial form inhabiting all seas from tide mark to
over 4000 meters. Most of them are littoral. Some prefer warmer seas. Shallow water
forms have fan- like structure. Deep sea forms branch in all directions. It forms one of
the chief attractions of the submarine gardens of shallow tropical and subtropical
waters.
Distribution: Found in South Atlantic and West Indies, Indo-Pacific ocean, Malay
Archipelago, Bermuda and Bahamas.
Comments: Commonly called as Sea fan, whose graceful form and soft colouration
constitute one of the chief attractions of ‘submarine’ gardens. It forms erect, yellowish
or reddish arborescent branches in one plane connected by cross connections in a
feathery manner and contour of the body becomes fan-like. Branches are united by
numerous cross-branches, forming a network with meshes 2 to 5 mm wide, calcareous
in nature. Base of the colony is expanded to form a hold fast organ called as pedal disk.
Polyps or anthocodia emerge from branches and contain tentacles, mouth and
gastrovascular cavity. Polyps occur in rows and on two sides only of the stem and its
branches. In magnified portions, holes for polyps are seen. Skeleton consists of
hornlike material called Gorgonin embedded in mesoglea. An axial rod extends
throughout the colony and its branches. Axial rod consists of medulla and cortex.
Gorgonin is proteinaceous consisting of tyrosine amino-acid. Sexes are separate.
SUMMARY
In this topic we have discussed some important specimens like- Hydra, Obelia, Aurelia,
Sea-anemone and Gorgonia under phylum Coelenterata. The Coelenterates are all most
marine, distributed generally throughout the seas. Aristotle considered Coelenterate as
the intermediate between plants and animals and termed them Acalephae or Cnidae.