POLYMORPHISM IN CNIDARIA(COELENTERATA) - HYDROZOA, SCYPHOZOA, ANTHOZOA
POLYMORPHISM
Definition
One of the important characters of Eumetazoa is the division of labour. In the vast majority, i.e., from flat worms to chordates, the division of labour involves the assignment of different functions to different parts or organs of the individual. In coelenterates, low organization and lack of organs do not permit any great degree of specialization for different functions within the limits of an individual. Therefore, in these animals the different vital functions are assigned to different individuals. This form of the division of labour is known as polymorphism. Polymorphism is, thus, the phenomenon of occurrence of an animal in more than one morphological and functional form. The individuals of polymorphic animals are known as the zooids or persons.2. Grades
Some coelenterates possess only two types of zooids, others have three types, and still others several types. These forms are respectively described as dimorphic, trimorphic and polymorphic. There are, thus, three grades of polymorphism.1. Dimorphic Forms.
The dimorphic forms have two types of individuals, namely, polyps or hydranths and medusae. These types are regarded as the fundamental types from which the additional types found in the trimorphic and polymorphic colonies are derived by modification.(a) Polyps. The polyps have a cylindrical form, are fixed by aboral end, enclose a wide gastrovascular cavity, and bear mouth and tentacles at the free oral end. They serve to feed the colony and are, therefore, also known as the gastrozooids or trophozooids.
(b) Medusae. The medusae have a bell, or bowl, or saucer-shaped body, lead free-swimming life when mature, enclose gastrovascular cavity in the form of narrow radial and circular canals, and bear mouth and marginal tentacles. They bear gonads and bring about sexual reproduction. They are, therefore, also known as the gonophores or sexual zooids.
Though very different in form and function, the polyps and medusae have a similar basic plan so much so that they can be derived from each other Bougainvillea (class Hydrozoa) and Pennatula (class Anthozoa) are examples of dimorphic colony.
(i) Bougainvillea (Fig. 4.4). The polyps are stalked and uncovered, have an elongated manubrium and bear a single circlet of solid, filiform tentacles. They feed the colony. Medusae have four perradial tufts of simple marginal tentacles and four groups of branched oral tentacles. They arise by budding from the hydrocaulus, polyp stalk and other medusae. They are sexual zooids.
(ii) Pennatula. Pennatula has two types of polyps autozooids that feed the colony and siphonozooids which maintain a current of water through the colony and also bear gonads.
(iii) Corallium (also an anthozoan) exhibits Pennatula-like dimorphism.
2. Trimorphic Forms.
The trimorphic forms have three types of zooids, i.e., polyps, medusae and gonozooids or dactylozooids. The polyps medusae are similar to those of dimorphic forms. The gonozooids are modified polyps. They lack mouth and tentacles. They serve to bud off medusae their morphological equivalents. The dactylozooids are also modified polyps. They are mouthless and serve to protect the colony.Examples of trimorphic colonies are many. Obelia and Millepora are well known.
(a) Obelia. The zooids of Obelia include polyps, gonozooids and medusae. The polyps are surrounded by hydrothecae and gonozooids by gonothecae. The medusae are saucer- like and bear gonads on radial canals.
(b) Millepora (Fig. 4.10 and 4.11). The zooids of Millepora include gastrozooids, dactylozooids and medusae. The gastrozooids have short, plump body with mouth and four knob-like tentacles. The dactylozooids have a long, slender body without mouth but with several, short, alternating, knobbed tentacles. The medusae develop in pits or ampullae of the colony and are greatly reduced, being without velum, mouth, tentacles and canals. They bear four nematocyst bearing knobs on the margin and gonads on the long manubrium.
3. Polymorphic Forms.
The polymorphic forms have several types of zooids. All these are modifications of the polyps and medusae. The best known polymorphic forms are Hydractinia and members of the order Siphonophora.(a) Hydractinia (Fig. 5.1 and 5.2). Hydractinia develops four types of zooids: gastrozooids, dactylozooids, gonozooids and sporosacs. The gastrozooids have the mouth and tentacles and feed the colony. The dactylozooids lack mouth and are defensive in function. They are further of two types :spiral zooids with short capitate tentacles and tentaculozooids which are long, slender and devoid of tentacles. The gonozooids retain short tentacles called the nematocyst heads. The sporosacs are reduced sac-like medusae. They produce gametes, either ova or sperms.
(b) Siphonophora. The siphonophores form free-swimming colonies with the highest degree of polymorphism. Their polyps occur in three modifications: gastrozooids, dactylozooids, and gonozooids.
(i) Gastrozooids. The gastrozooids are also called the siphonozooids, hence the name of the order. They feed the colony. They have the usual polyp form but lack the usually located tentacular ring. Instead, they bear a single, hollow, long and contractile tentacle at or near the base. The tentacle gives off lateral branches, the tentilla, each ending in a knob or coil of nematocysts.
(ii) Dactylozooids. The dactylozooids are also called the palpons, feelers or tasters. They lack mouth and their basal tentalce is unbranched. They may be long, hollow and tentacle-like, when they are termed the tentaculozooids.
(iii) Gonozooids. The gonozooids may look like the gastrozooids and even have a mouth, but lack a tentacle. Usually, however, they are long slender and branched and are called gonodendra. They may bear tentacle-like dactylozooids called the gonopalpons. The gonozooids produce clusters of gonophores on them.
The medusoid individuals exist in four modifications : swimming bells, bracts, gonophores and pneumatophore (Fig. 5.3).
(a) Swimming Bells. These are also termed the nectophores, or nectocalices. They are medusae with velum and radial and circular canals, but without mouth, manubrium, gonads and sense organs. They are very muscular and bring about locomotion of the colony.
(b) Bracts. These are also called the hydrophyllia or phyllozooids. They are thick, gelatinous individuals with simple or branched gastrovascular cavity. They are protective in function.
(c) Gonophores. The gonophores may be medusa-like but lack mouth, tentacles and sense organs. They may be reduced to rounded sacs. In some cases, the female gonophores are medusa-like and male gonophores are sac-like. The gonophores produce gametes.
(d) Pneumatophore. It is also known as the float. It is an inverted medusa without mesogloea. In outer (exumbrellar) and inner (subumbrellar) walls are respectively called the pneumatocodon and pneumatosaccus or air-sac. The opening of the air- sac is directed upward, is reduced to a small pore, the pneumatopore, and is guarded by a sphincter muscle. At the bottom (original roof) of the air-sac, the epidermis is modified into a gas gland that secretes gas having composition similar to that of air. The float keeps the colony afloat.
All types of zooids arise by budding from a common stem or coenosarc. The latter may have the form of a tube or disc and bears zooids in groups called cormidia.
The common polymorphic siphonophores are Halistemma, Physalia, Velella and Porpita.
● Halistemma (Fig. 5.4). Halistemma possesses a long, slender stem with a small float at the top, several close-set swimming bells below the float and numerous cormidia lower down. The cormidia arise from nodes and each comprises a gastrozooid or dactylozooid and several sporosacs. Bacts arise from the internodes and partly cover the sporosacs.
● Physalia (Fig. 4.12 and 4.13). Physalia has a disc-like coenosarc with a large, sail- bearing, balloon-like float over it and several cormidia hanging from it. A cormidium consists of gastrozooids, small and large dactylozooids, gonodendra bearing gonophores, gonopalpons and peculiar gelatinous zooids of unknown function. The colony is poisonous.
• Velella (Fig. 4.15). Velella has a rhomboidal float divided internally into a number of gas-filled chambers. It bears an oblique sail above and a disk-like coenosarc beneath. There is a single, large, central gastrozooid surrounded by gonozooids which are, in turn, surrounded by dactylozooids. The gastrozooid lacks a tentacle. The gonozooids have mouth and bear gonophores at the base. Dactylozooids are tentacle-like.gastrozooid surrounded by gonozooids which are, in turn, surrounded by dactylozooids. The gastrozooid lacks a tentacle. The gonozooids have mouth and bear gonophores at the base. Dactylozooids are tentacle-like.
• Porpita (Fig. 4.17). Porpita resembles Velella, except that it lacks sail and has a circular float.
3. Forms of Polymorphism
Polymorphism has two main forms: metabolic and reproductive. Metabolic form is represented by the zooids having vegetative functions such as feeding, swimming and defence. It includes gastrozooids, dactylozooids, pneumatophores, nectocalyces, etc. All these individuals are polypoid. Reproductive form of polymorphism is represented by the zooids having reproductive function. It includes the asexual zooids, such as blastostyles or gonozooids, and sexual zooids, such as gonophores or medusae. The former are polypoid and the latter medusoid.4. Nature of Polymorphic Forms
There are two views about the nature of polymorphic forms: polyorgan theory and polyperson theory.1. Polyorgan Theory. This theory, advanced by Huxley and others, considers a polymorphic form a single individual and the various zooids its organs modified for different functions. The theory suggests two alternatives : polyorgan hydriform theory and polyorgan medusiform theory which hold that the parts of a polymorphic form are modified organs of a polyp or a medusa respectively.
2. Polyperson Theory. This theory, put forward by Leuckart, regards a polymorphic form a colony having many individuals or zooids modified for different functions. This theory also offers two alternatives : polyperson hydriform theory and polyperson medusiform theory which propose that the different zooids of the colony are modified polyps or modified medusae llium respectively.
It is obvious that at least certain parts of a siphonophore, such as gastrozooids and gonophores, cannot be homologised to parts of a polyp or of a medusa. Moreover, if the entire siphonophore is equivalent to an individual polymorphism does not exist. Most zoologists consider siphonophore a colony, possibly of more than one generations. The first generation is represented by the float that arises from the planula, second by other zooids and third by gonophores.
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