Q.1 Write the classification of phylum coelenterata upto classes and give its main characters with suitable examples. (2006)
Related Questions -
Q. Giving salient features of Phylum Coelenterata classify it upto classes, with suitable examples. (2014)
Ans. General Characters of Coelenterata: -
(1) It has a cell – tissue grade of body organization with most of the cells scattered and specialized for different functions.
(2) All aquatic, some in fresh water but mostly marine.
(3) Solitary or colonial, sedentary or free swimming.
(4) Radial or biradial symmetry about a longitidinal oral-arboral axis.
(5) Two types of individuals, free or attached polyp and free swimming medusa.
(6) Body wall diploblastic with two cellular layers, outer epidermis and inner gastrodermis with a gelatinous mesoglea in between.
(7) The inner gastrovascular cavity often branched and divided by mesenteries (septa)and with only one opening themouth which is often encircled by short and slender tentacles.
(8) One or both body layers with peculiar stinging cell oraganelles called nematocysts which are very abundant on tentacles.
(9) Nerve net present, sensory organs simple or complicated. Some with ocelli and statocysts.
(10) Muscular system of epithelio and endothelio muscle cells.
(11) Digestion intra as well as extracellular.
(12) Asexual reproduction by budding or fission, sexual reproduction by gametes.Sexual forms monoecious or dioecious, development exhibits planula larva.
(13) Life history commonly illustrates alternation of generation or metagenesis in which a sexual, free medusoid generation alternates with an aseual, sessile usually colonial polypoid generation.
CLASSIFICATION: -
It is divided into following classes-
Class 1 : HYDROZOA: -
(Hydra-water,zois-animal)
(i) Solitary or colonial.
(ii) Either only polyps or asexual polyps and sexual medusae.
(iii) Hydranths without pharynx and septa or mesenteries.
(iv) Medusae with true velum.
(v) Gonads epidermal.
(vi) Freshwater or marine, sessile or free.
Class 2 : Scyphozoa: -
(Skyphos-cup,zois-animal)
(i) Solitary and medusoid
(ii) Polypoid stage reduced or absent.
(iii) Bell or umbrella shaped medusa without velum.
(iv) Much enlarged gelatinous mesoglea with fibres and cells.
(v) Margin of bell with eight notches bearing sense organs.
(vi) Exclusively marine.
Class 3 : Anthozoa: -
(anthos-flower, zois-animal)
(i) Solitary or colonial .
(ii) All poyps no medusae.
(iii) Mouth leads into pharynx or stomodaeum.
(iv) Gastrovascular cavity subdivided by eight or less or more septa or mesenteries.
(v) Mesenteries with nematocysts and gonads.
(vi) Exclusively marine.
Q.2 Give an account of polymorphism in coelenterata. (2007, 17)
Related Questions -
Q. Write an essay on the polymorphism in Coelenterata. (2012)
Q. Write short note on polymorphism in obelia. (2013, 14)
Q. Write short note on Polymorphism in Coelenterata. (2018)
Q. Describe polymorphism in coelenterata. (2019)
Ans. Meaning of Polymorphism: -
Occurrence in the same of more than one type of individuals, which differ in form and function is known as polymorphism. This ensures an efficient division of labour between several individuals.
Polymorphism is an important feature of hydrozoan colonies which provide some of the best examples.
Basic Forms: -
(1) Polyps: -
A polyp has a tubular body with a mouth surrounded by tentacles at one end. Other end is blind and usually attached by a pedal dis to the substratum.
(2) Medusa: -
A medusae has a bowl or umbrella shaped body with marginal tentacles and mouth centrally located ona projection (manubrium)of the lower concave surface.
Although polyps are typically sessile and medusae are generally motile, there exists a homology between the two in their basic features.
Importance of Polymorphism: -
Polymorphism is essentially a phenomenon of division of labour. Different functions are assigned to different individuals, rather than to parts or organs of one individual. Thus polyps are concerned with feeding,protection and asexual reproduction, while medusae are concerned with sexual reproduction.
Patterns of Polymorphism: -
Degree of polymorphism varies greatly in different groups of hydrozoa.
(1) Dimorphic: -
Simplest and commonest pattern of polymorphism is exhibited by manyhydrozoan colonies like Obelia, Campanularia etc. They have only two types of zooids.
(a) Gastrozooidsor hydranths are concerned with feeding.
(b) Gonozooids or blastostyles are concerned with asexual budding forming sexual medusae or gonophores. Such colonies bearing two types of individuals are called dimorphic and the phenomenon is termed dimorphism.
(2) Trimorphic: -
Some forms like Plumularia are trimorphic. Besides gastrozooids and gonozooids they also posses a third type of individuals the dactylozooids. These are functionally non-feeding and defensive polyps bearing batteries of nematocysts.
(3) Polymorphic: -
Coelenterates having more than three types of individuals are called polymorphic. A somewhat greater degree of polymorphism is found in the colony of hydractinia with five types of polyps. These are -
(i) Gastrozooids - for feeding
(ii) Spiral dactylozooids - for protection
(iii) Long, sensory tentaculozooids with sensory cells
(iv) Skeletozooids-as spiny projections of chitin and,
(v) Gonozooids reproductive being male or female, bearing medusae for sexual reproduction.
Extreme examples of polymorphism are seen in the pelagic or swimming colonies of the order siphonophora and chondrophora. As in hydractinia both polypoid and medusoid individuals, specialized for various vital functions, occur in the same colony. Polymorphism reaches its peak in siphonophora.
(a) Modificatoions of Polyps: -
Polypoid individuals include -
(1) Gastrozooid or feeding polyp with a mouth and a long tentacle.
(2) Dactylozooid or protective polyp without mouth and usually with a long basal tentacle.
(3) Gonozooidor reproductive polyp which produces sexual medusae or gonophores.
(b) Modifications of Medusae: -
The medusoid individuals are of the following types -
(1) Nectophore or nectocalyx or swimming zooid with a muscular bell without manubrium or tentacles.
(2) Pneumatophore or float as a bladder like medusa filled with secreted gas.
(3) Phyllozooid or bract usually leaf like and studded with nematocysts serving for protection of other zooids.
(4) Gonophore bearing gonads which may be either male producing sperms or female producing ova.
Q.3. Write shorte note on obelia medusa. (2005)
Other Related Question -
Q. Write short note on obelia colony. (2008)
Ans. Obelia Medusa (Or Obelia Colony) : -
The asexually reproducing polyp colony. During this stage of life, Obelia are suspended to substrate surfaces. On this mature colony there are individual hydranths called gastrozooids, which can be found expanded or contracted, to aid in the growth of this organism by feeding; and the reproductive polyp gonozooids that have medusa buds. Other hydranths are specialized for defense. The main stalky body of the colony is composed of a coenosarc, which is covered by a protective perisarc.
The next generation of the life cycle begins when the medusae are released from these gonozooids, producing free swimming male and female medusae velum with gonads, a mouth, and tentacles. The physical appearance of the male and female medusae velum, including their gonads, are indistinguishable, and the sex can only be determined by observing the inside of the gonads, which will either contain sperm or eggs. The medusae reproduce sexually, releasing sperm and eggs that fertilize to form a zygote, which later morphs into a blastula, then a ciliated swimming larva called a planula.
Structure: -
Obelia consist of two types of body plans of a polyp and medusa in their polymorphic life cycle. They are diploblastic, with two true tissue layers - an epidermis, gastrodermis, and a jelly-like mesoglea filling the area between the two true tissue layers. They carry a nerve net with no brain or ganglia. A gastrovascular cavity is present where the digestion starts and later becomes intracellular. They have incomplete digestive tracts where the food enters, digests and is 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.