Kingdom Animalia Cnidaria, Biology tutorial

Kingdom Animalia Cnidaria

Characteristics of Kingdom Animalia Cnidaria:

The wide-ranging characteristics of Cnidaria are as follows:

Cnidarians are diploblastic. This signifies that the body wall is composed of 2 layers of cells: outer ectoderm and inner endoderm. Both layers are divided by gelatinous structure-less layer known as mesogloea. This mesogloea may though have cells that migrate from other layers. The diploblasts can be explained as getting tissue level of organization. This is due to 2 layers of cells are exact and carry out stable functions. Endoderm cells are feeding cells from which nutrients disperse to ectodermal cells that are liable for gaseous exchange and from that gas can disperse to endodermal cells. Presence of 2 layers of cells and structure-less mesogloea are feature features of the group. Cnidarians as well have the radial symmetry. Therefore means that it can be slice into 2 equal halves, across any diameter. Animals which illustrate the radial symmetry are inclined not to more about. In its place it is those which have bilateral symmetry which have good adaptation for locomotion. Bilateral symmetry provides the more compact and streamlined shape and permits the greater specialization of parts of body.

Essentially the bodies of cnidarians are sac-shaped with only 1 opening that serves both for ingestion and egestion. This opening, "mouth" leads to the cavity within sac-shaped body known as enteron in which digestion takes place. Few cnidarians are sessile (polyps) whereas others (medusac) swim freely.

The Nature, Form and Ecological Adaptations of the Hydra:

Hydra belongs to class of cnidarians known as hydrozoa. It is the fresh water coelenterate. It contains the thin supple, hollow body approx 1cm long that is fixed to the substrate by the sticky secretion made by basal disc, or foot. Opposite (distal) end is the conical projection (known hypostome) at apex of which is mouth. From base of hypostome, extends the whorl of tentacles. Similar every coelenterates, wall of hydra comprises of 2 layers of cells arranged back to back and divided by the thin layer of colloidal mesoglea. Two layers of cells are composed of mostly musculoepithelial cell that is cuboidal in shape. Every cell has the nucleus. It connects with neighboring musculoepithelial cell to create the epithelium which forms both inner and outer layers of body. Basal part of the cell spread out, at tangent with cell axis and has small muscle fibres. Basal muscle fibres of epidermal cells run parallel to aboral-oral axis whereas that of endoderm run in the spherical or transverse plane. Thus epidermal ones contract to shorten animal while those of gastodermis (cells lining endoderm) elongate it.

Hydra has the nervous system composed of neurons that are significantly lengthened and are joined with each other. They are much more at hyposture where control activity of tentacles. Epidermis has the diversity of stinging cells known nematocysts that assist animal to capture food organisms. Small cells discovered at base of epidermis assist to replace cells. They are known as interstitial cells. Epidermis is composed of 5 kinds of cells:

i) Musculoepithelial cells.

ii) Nematocysts or stinging cell

iii) Interstitial cells - not specialized and can replace others.

iv) Neurons.

v) Sensory cells - specialized for special function.

In gastrodermis musculo-epithalial cells that form main cells here are more flexible. They are prepared with flagellum at the free ends. They require these to wave in food particles. In addition, there are small brownish or greenish algae which succeed inside hydra. There are several glandular cells full of secretory granules in hypostomal region and sprinkled in between muscuo-epithelia cells of gastrodermis. Sex cells, ova and spermatozoa are made by epidermal cells under special.

Cell Differentiation and Specialization:

There are less than 10 different kinds of cells in hydra. These cells, whether together or independently execute their individual functions. Hydra, although simple, illustrates all grades of cell specialization. Individual cells contain characteristic features for definite functions. Instances are sense cells. Specific cells performing same functions are clustered together and are mutually dependent. Instances are sense cells that are sensitive to touch; absorbing cells for absorbing digested food, stunging cells, for capturing hydra's food; glandular cells for digesting food; and nerve cells for managing work of other cells. All of these are groups of cells of same kind doing one job. This is the tissue level of organization. Hydra, although multicellular, and distinguished, doesn't contain specialized organs similar to most other coelenterates of its kind. Other multicellular animals contain special organs or systems that perform each life progression. In person for instance, kidneys are for excretion, ears for hearing and so on.


It is active as illustrated by fact that it moves by looping and somersaulting. These are the result of contraction and relaxation of the column and attachment of tentacles to substrate; releasing and reattaching them.

Reply to Stimuli:

Hydra can develop its mouth to numerous times size of its own diameter to consume prey. Presence of few complicated amino acids in many animal tissues can cause it to open the mouth to such the extent as to turn inside out.


There is the constant chain of chemical actions resulting in procedures such as assimilation, utilization, digestion and excretion; all of which are connected.


Through development and constant self renewal, hydra holds its typical shape. It also contains high power of regeneration if divide into parts, it can redevelop missing part for form the whole hydra in the few days.


When well fed, hydra makes projections from the body about two thirds down the column. This bulge is composed of 2 layers of cells. These are buds that turn out to be young hydra joined to mother hydra. They seem youngest always on top and oldest further below until they are disconnected.

When divide into 2 it can redevelop lost longitudinal half. Irregularly it performs this on its own by making 2 mouths from one, each shortly enclosed by its set of tentacles, and slowly obtaining its own body (or column). All of these are proof of development and rebuilding of parts. Dissimilar from what seems to be asexual reproduction, there is sexual reproduction. Under diverse circumstances of temperature and pressure, dissimilar species of hydra turn out to be sexual. They make from their interstitial cells circular protrusions having eggs and conical ones having sperms. By the time eggs are fertilized, hydra disintegrates. Only fertilized egg survives to carry on species. Understanding simple type of hydra and few numbers of cells is significant.

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