Amongst the coelenterates, hydrozoans provide very good examples of polymorphism. The phenomenon is essentially for division of labour. Division of labour is first seen in the cells of Hydra where the cells are specialized to perform different functions of individual as a whole. Physiological differentiation of this type had its effect upon the morphology of cells which led to cells specialization and give rise to cells of different structures. In Obelia this specialization is earned still further. In it not only cells are specialized but individuals get specialized to perform different functions. The polyp performs different functions. The polyp performs vegetative function such as feeding, respiration, etc. and the free swimming medusae are reproductive nature.
There are different types of polypoids and medusoids specialized for different functions. There are three types of polypoid and four types of medusoids individuals as given below :
I. Popypoid Zooids
The gastrozooids or the siphons are the nutritive or food-ingesting individuals of the colony. Each gastrozooid is a tubular or seccuLar structure with a large mouth. A single, long, contractile and hollow tentacle arises from the base of the gastrozooid. It bears numerous lateral contractile branches called the tentilla, each ending into a knob or coil of nematocytes.
These are the protective polyps of the colony and are variously known as palpons, tasters or feelers. Typically they resemble the gastrozooids except that they lack a mouth and their basal tentacle is unbranched. In Vallela and Porpita the dactylozooids arise from the margin of the colony in the form of long, hollow and tentacle-like fringing bodies called tentaculozooids. When associated with gonophores, the tentacle-like dactylozooids are known as gonopalpons. In Physalia the dactylozooids become excessively long.
They are reproductive zooids which are also known as blastostyles. They are without mouth and tentacle. They reproduce asexually by budding and form medusae. In Vallela and Porpita, they resemble a gastrozooid and possess a mouth. Usually, the gonozooids take the form of branched stalks, called the gonodendra. These bear grape-like clusters of gonophores and are often provided with gonopalpons as in Physalia.
II. Medusoid Zooids
1. Swimming Bell
The swimming bells which are also known as nectocalyces, nectophores or nectozooids are medusoid form with a bell, velum, four radial canals and a ring canal. But these are devoid of mouth, manubrium, tentacles and sense organs. Its shape is variable and may be bilaterally symmetrical, prismatic, elongated or flattened. Due to well developed musculature, swimming bells act as excellent swimming organs and help in the locomotion of the colony.
The pneumatophores or the floats are bladder or vesicle-like structures filled with gas, and keep the colony floating. Each pneumatophore represents an inverted medusa bell, devoid of mesogloea and consisting of an external exumbrellar wall, pneumatocodon, and an internal subumbrellar wall, the pneumatosaccus or air-sac. The walls of both these are double-layered and are highly muscular. The space between the two walls is known as gastrovascular cavity.
A great degree of variation in shape and size is observed in different siphonophores. In Agalima, the float is simple and its air sac is lined by a layer of chitin secreted by the epidermis.
The shape of pnematophore is variable and may or may not be divided into a number of concentric chitinous chambers arranged in one plane. These communicate with each other and with the central chambers by pores in their walls. The air sac may open or closed. The air sac may be perforated by a single or several pores. In some cases a portion of the float is partly constricted off and assumes the form of an ovoid medusa-like, called aurophore.
The bracts which are also known as the phyllozooids or hydrophyllia are thick, gelatinous and curved plates of mesogloea. These may be prism-like, leaf-like, shield-like or helmet like in appearance. They are unlike meduase and contain a simple or branched gastrovascular canal.
The gonophores or the reporductive medusoids occur singly on separate stalks or in clusters on polypoid gonozooids as in Velella or on simple or branched gonodendra. The gonophores may be medusa-like with bell, velum, radial canals and a manubrium bearing gonads. But the mouth, tentacles and sense organs are always absent. In number of hydrozoans e.g. Physalia, the female gonophores are medusa-like while the male ones are sac-like. In animals, like Physalia (male), the gonophores may remain attached to the colony or are set free as in female Physalia, Porpita and Velella. Since they cannot feed, they perish after the discharge of sex-cells. The gonophores are dioecious but the colonies are hermaphrodite bearing both types of gonophores in the same or separate clusters. Gonophores may be budded off from the pedicel of the gastrozooid as in Diphys, or from a blastostyle as in Velella or from coenosarc as in Agalmopsis.
Types of colonics in siphonophora
Hydrozoa belonging to the orders siphonophora and chondrophora are exclusively marine, planktonic or pelagic and polymorphic colonies. The colonies are usually delicate, transparent and beautiful The zooids in each colony are attached to the coenosarc or coenosome and their arrangement exhibits diversity in different colony. Because of great diversity of colonics in siphnophora and chondrophora can be illustrated only by some examples of the colonies such as Halistemma, Physalia and Porpita.
(A) Halistemma: It belongs to suborder Physophorida of the order Siphonophora. This genus occurs in mediterranean and other seas. The colony exhibits great degree of polymorphism, coenosarc is modified into a long, slender, floating axial tube or stem, to which polymorphic zooids are attached all along its length. The upper end of the stem is expanded to form small pneumatophore filled with gas. It serves to keep the colony’s right side up in water. Below the pneumatophores are several closely set transparent swimming bells or nectocalyces which contract rhythmically to take in water and pump it out, thus propelling the colony through water. Below the last nectocalyx on the stem, at regular intervals are similar groups of closely set individuals cormidia together forming a linear cormidium.
Each group of cormidium consists of four types of zooids (i) tubular gastrozooids with mouth and long branched tentacle bearing batteries of nematocysts (ii) mouthless dactylozooids with an unbranched sensory tentacle (iii) shield-shaped leaf hydrophyllium or bract which partially covers and protects the cormidium and (iv) unisexual gonozooids or sporosacs bearing male or female medusae or gonophores. Halistemma shows bilateral symmetry. Its life history includes a ciliated planula larva, one end of the planula invaginates to form pneumatophore while the other lower end forms polyp. The first polyp by elongation and budding forms the colony.
(B) Physalia: Physalia is a polymorphic colony of bright blue colour belonging to suborder Physophorida of the order Siphonophora. It is one of the most dreaded and most beautiful pelagic coelenterate found floating in tropical and subtropical oceans. The members of the colony develop from coenosarc. It has, on the aboral side a large gas-filled pneumatophore formed by several medusoids. It floats above the surface of water. Dorsally the pneumatophore forms a crest or sail for manoeuvering the wind. It contains a gas gland which secretes a gas having the similar composition as that of air, but with a large proportion of carbon monoxide. Below the pneumatophore hangs a colony of several non-linear cormidia. Each cormidium is a group of polymorphic individuals which are modified polyps such as gastrozooids, dactylozooids and gonozooids. Gastrozooids are tubular with mouth. Each gastrozooid possesses a long tentacle which may hangdown several metres below the pneumatophore or float. The gastrozooid feeds on fish. It applies its lips to the fish and partially digested food is sucked in liquid form. Dactylozooids are of various sizes. Each is a tubular mouthless individual with a long tentacle (may reach a length of 30 metres) having strong muscles and twisting ribbon of ncmatocysts. They form a drift net for capturing fish for food which is drawn upto the gastrozooid by the contraction of tentacles.
These tentacles can inflict serious and sometimes fatal injuries to man. Swimmers can be severely poisoned or even paralysed by contact of these tentacles. Gonozooids or gonodendra are
branching blastostyles having leaf-like gonopores and male and female medusae or gonophorea of deep blue colour. The male mcdusoids are reduced and remain attached but the female mcdusoids become detached and swim freely. Pneumatophores and gonophores are modified medusoids. The members of the colony show a division of labour.
(C) Porpita: It is a polymorphic colonial hydrozoa belonging to order chondrophora. The colony resembles a medusa. It has a large disc-like body with a chambered, chitinous aboral pneumotophore containing air. Each chamber communicates with exterior by two pores. The stem is shortened to a flat coenosarc. From the disc-shaped body hangs ventrally a single large central gastrozooid bearing mouth and stomach of the colony.
Surrounded by cluster of closely set tubular gonozooids or blastostyles, each having mouth and bearing medusae. The rim of the disc bears numerous dactylozooids with nematocysts.
A thick cellular mass the so-called liver is present in between the gastric cavity of gastrozooid and the pneumatophore. This cellular mass is believed to have an excretory function. It is traversed by two systems of canals. The endodernal canals communicate with the enteron of the zooids throughout the colony. The ectodermal or tracheal canals, internally lined by chitinous layer, communicate through the air chambers with the exterior. Air chambers thus perform the function of respiration. Nectocalyces are absent in this colony.