What is polymorphism? polymorphism in cnidaria/ polymorphism in siphonophora


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What is polymorphism? polymorphism in cnidaria/polymorphism in siphonophora


In biology the term polymorphism can be defined as the phenomenon of existence of different physiological and morphological forms represented by an extensive range of variation within a single species.

Since cnidaria or siphonophora exhibit two different types of body forms (i.e., polyp and medusa), hence modification of the two are given below:

What is polymorphism? polymorphism in cnidaria/ polymorphism in siphonophora

Modifications of the Polyp:

A. Gastrozooids:


1. They are commonly called as trophozooids or nutritive zooids.

2. They are like a typical polyp.

3. Their body structure is cylindrical and funnel-shaped with a large mouth.

4. They have a well-formed coelenteron.

5. They have a single long contractile and hollow tentacle which arises from the base of the gastrozooid. The tentacles may be provided with batteries of nematocysts (e.g., Apolemia).

Examples: Hydractinia, Halistemma, Apolemia, Physalia.

Function: Those zooids are meant for nutritive or feeding.

Modification: The gastrozooid exists in the following modified forms:

1. Siphon:

(i) They are the only member of the colony which can ingest food.

(ii) It is a polyp form, but without normal tentacles.

(iii) They have a single large hollow tentacle.

(iv) Their tentacle is highly contractile and hangs from or near the base.

(v) This tentacle bears lateral elongated contractile branches, called tentilla.

(vi) Each tentilla in them terminates in a large and complicated knob or coil of nematocysts.

(vii) In some cases the tentacles are absent too (e.g., Velella).

2. Siphonozooid:

(i) In Pennatularia the gastrozooids are modified to produce a current-produce device at the expense of other structures.

(ii) They usually does not have tentacles, longitudinal muscles but may sometimes possess septal filaments.

(iii) In some rare cases, the zooid may possess a single tentacle.

(iv) They may remain scattered or may be limited to rachis.

(v) They are usually located on the dorsal side of the rachis in between the leaves.

(vi) They might be arranged in clusters as in Milk.

(vii) They are again modified as mesozooid and autozooid.

B. Dactylozooids:


1. They are also called tasters or feelers or macrozooids.

2. These are actually derived from the gastrozooids by the reduction or total loss of mouth.

3. They are elongated and highly extensible.

4. They have a single basal tentacle which is usually unbranched and capitate.

4. They are provided with nematocysts and adhesive cells.

Function: These are protective zooids.

Examples: Hydractinia, Halistemma, Physalia.

Modification: The dactylozooids exhibit following structural variations:

1. Tentaculozooids:

(i) In Hydractinia, the dactylozooid shows a long tentacle-like appearance.

(ii) They are definitely arranged in relation to gastrozooids and are usually situated at the margin.

(iii) In Velella and Porpita, the margin of the colony bears long, hollow and tentacle-like defensive dactylozooids called tentaculozooids.

2. Spiral Zooids:

(i) In Hydractinia, the spiral zooids (dactylozooids) with capitate tentacles remains scattered throughout the colony.

(ii) These are tubular shaped zooid with cnidocytes and adhesive cells.

3. Sarcostyles or Nematophores:

(i) In Plumulariidae, the sarcostyles spring from tiny thecae (nematothecae) located on the stems and on the Hydrothecae (usually three to each) of the gastrozooids.

(ii) They usually have club or capitate ends, beset with nematocysts or adhesive cells or both.

4. Palpons:

(i) The palpons in Chondrophora contains simple, hollow, tentacle -like bodies which spring from the margin of the body.

(ii) They act as the dactylozooids in Velella and Porpita.

(iii) In these two forms the palpons may remain associated with gono-phores in these two above men- Honed forms, called gonopalpons.

5. Cyston:

(i) Cyston is a distal pore present in the dactylozooid in siphonophores.

(ii) It plays an important function in excretion.

6. Dactylozooid of Millepora:

These are dactylopores from which project long, filamentous, mouth- less dactylozooids with irregularly disposed tentacles on the surface of the body.

C. Gonozooids:


(i) They are also called as blastostyles or gonangia.

(ii) They are the reproductive zooids of the colony.

(iii) Their body structure is club-shaped without mouth and tentacles.

(iv) Coelenteron is greatly reduced in them.

(v) They may be dioecious or monoecious.

(vi) The living tissue in them is called blastostyle.

(vii) These are enclosed by gonotheca.

Examples: Velella, Physalia.

Function: Reproduction.

Modification: The typical form becomes modified in different members.

1. Gonosiphon:

(i) These are the gonozooids which may resemble gastrozooids and may even possess a mouth in Velella and Porpita.

(ii) The tentacles are usually absent in them.

2. Gonodendron:

(i) Gonodendrons are present as branched stalks which bear grape-like clusters of gonophores, in Siphonophora.

(ii) They are usually provided with a long retractile gonopalpon.

3. Gonopalpon:

In Siphonophora, They are tentacle-like dactylozooid which remains associated with the gonophores in Siphonophora and is tn termed as gonopalpon.

D. Special Types of Zooids:

(1) Gonostyles:

(i) They are also called Secondary Siphonozooids.

(ii) In Porpita, they are primarily reproductive in function and may secondarily be nutritive. 

(iii) The mouth and coelenteron are present in them. 

(iv) The gonads remain attached with the siphonozooids.

(2) Hydrorhiza:

In Obelia, they (hydrorhiza) acts as the organ of attachment for the whole colony.

(3) Hydrocaulus:

In Obelia, the hydrocaulus arises from the hydrorhiza which bears different zooids and helps to convey the food matters to the different parts of the colony.

Modifications of Medusoid Form:

A. Nectocalyx:


(i) It is also called Nectophore or Swimming Bell.

(ii) It is a medusoid form which have a bell, velum, 4 radial canals and a ring canal.

(iii) The mouth, manubrium, tentacles and sense organs are absent in them.

(iii) They are bilaterally flattened, may be prismatic or may be elongated.

(iv) In bilateral forms two of the four radial canals take sinuous courses.

(v) The musculature of their body is well-developed which helps them in locomotion.

(vi) This form is usually present in most Siphonophores except physalia.

Examples: Muggiaea, Diphyes, Halistemma.

Function: Maintaining position in water and helps in locomotion. The contraction of nectophore in Nectalia also functions in jet propulsion.

B. Bract:


(i) They are also called Hydrophyllium or Phyllozooid.

(i) They do not resemble the medusa, though they are actually medusoid in origin.

(ii) They are thick gelatinous, prismatic or leaf-like or helmet-shaped in appearance.

(iii) Both mouth (except in Agalma) and tentacles are absent in them.

(iv) Coelenteron in them is either simple or branched.

(v) These forms are present in Siphonophora.

Function: It play an important function in providing protection.

C. Pneumatophore or Float:


(i) They are the inverted medusoid bells without mouth and tentacles.

(ii) They usually do not have mesoglea and is lined with an ectodermal layer.

(iii) In Halistemma, Physalia, there is usually one gas filled, bladder-like apical float or pneumatophore in a colony.

(iv) In Nectalia, the float is relatively smaller in size, oval shaped and contains a large branched gas gland. 

(v) In Velella, the float is flattened.

(vi) Their external exumbrellar side is called pneumatocodon and an internal subumbrallar side is called pneumatosaccus or air sac.

(vii) The original opening of their air sac is actually directed upward.

(viii) This opening in them may either be closed or reduced to a pore guarded by a sphincter muscle.

(ix) The air sac in them is usually lined by a chitinous layer which is secreted by the ectoderm.

(x) At the bottom of the air sac they usually have an expanded chamber termed as trichter or funnel.

(xi) The ectodermal lining of the trichter in them is modified into a gas gland.

(xii) The air sac of them is filled up with oxygen, nitrogen, argon, etc., but in physalia it contains a high proportion of carbon monoxide.

(xiii) This type is present in most siphonophores [e.g., Physalia, Halistemma, Agalma] except Diphyes.

Function: It is hydrostatic in function thus helps the colony to float in water.

D. Aurophores:


(i) It is usually ovoidal in shape.

(ii) A part of the pneumatophore becomes partially constricted off to form a bell-like aurophore which remains amongst the nectophores (e.g., Stephalia).

(iii) They remains in communication with the pneumatophore as well as with the exterior.

Function: They play an important function in swimming and excretion.

E. Gonophores or Spore sacs:


(i) They are the reproductive zooids and may either occur singly or in clusters.

(ii) They usually look like medusae being having bell, velum,  radial canals and manubrium.

(iii) Gonads in them are situated on the manubrium.

(iv) Mouth, tentacles and sense-organs are absent in them.

(v) They themselves are dioecious but the colony is hermaphroditic.

(vi) The female gonophores are medusa-like but the male gonophores are sac-like.

(vii) In some members of Hydroidea, the gonophores are sessile.

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