Hydra: Classification, Occurrence, External Morphology, Budding, and Regeneration


Kingdom: Animalia

Phylum: Coelenterata

Class:  Hydrozoa

 Order: Hydroida

Genus: Hydra

Species: Vulgaris


Coelenterata (or Cnidaria) is a phylum of lower invertebrates that includes acoelomates and radially symmetrical invertebrates (Radiata). Polyps and medusae are two morphologically distinct forms of coelenterate creatures. Polyps are sessile with a tubular body (e.g. Hydra), whereas medusae are free-swimming with an umbrella or bell-shaped body (e.g. Aurelia, Metridium). Hydra is a member of the Coelenterata phylum’s most primitive class, Hydrozoa. It has a simple shape and structure, making it a useful starting point for those learning about coelenterate organisation. Hydras come in a variety of forms. The following description applies to H. Vulgaris, a common Indian species, as well as other hydras species in general.

Hydra’s body is tube-shaped. The head is on one end, with tentacles surrounding the mouth/anusopening. The other end is referred to as the foot, and it contains the basal disc, which seals the tube. The stomach area is in the centre. The ectoderm and endoderm are separated by an extracellular matrix called the mesogloea in the body wall. Body parts formed by transverse sectioning regenerate the head from the apical end and the foot from the basal end of the body wall between the head and the foot. When tissue from different body levels is transplanted laterally to a host animal, it has varying abilities to convert into a head or a foot, respectively. In such transplantation trials, tissue acquired from a more apical position combines a higher capacity to generate a head with a lesser capacity to form a foot. A scalar tissue attribute (Gierer et al., 1972) known as positional value (Wolpert, 1969; Wolpert et al., 1974) or source density determines the tissue’s polarity and the graded distribution of the stated capacity (Gierer and Meinhardt, 1972). The positional value, or source density, of a Hydra, has its maximum value at the apical end and its lowest value in the basal disc by definition.


Hydras are worldwide in distribution and are solitary, sessile freshwater animals. They can be found in weed-infested lakes, ponds, streams, and seasonal ditches. They aren’t found in nasty or very warm water, but they thrive in cool, clean, and somewhat permanent and stagnant water. They are commonly seen clinging to and hanging downwards from the underside of solid objects in the water, such as leaves, sticks, stones, weeds, and so on. When they’re hungry, their body and tentacles are stretched to their utmost length and swayed in the water to catch any prey that comes close. When the body is disturbed, it instantly compresses into a tiny jelly-like bump that is invisible to inexperienced onlookers. It crumbles into a mushy, shapeless lump when removed from the water.

External Morphology


Hydra has a tubular or cylindrical body and resembles a polpy-like or polypoid coelenterate. It becomes elongated and narrow when completely extended, and it never extends beyond 1 cm in length. When the body is retracted, it is shorter and spherical, measuring only a few millimetres in length. Body symmetry is usually radial, with an oral-aboral axis and concentrically placed sections.


H. Vulgaris is a colourless Indian species. H. gangetica is a white or pink fungus found in ponds near the Ganges River. Pelmatohydra oligactis, often known as H. fusca, is a brown-coloured species. The brilliant green colour of Chlorohydra viridissima, formerly known as H. Viridis, is owing to the presence of symbiotic zoochlorellae Chlorella Vulgaris, a unicellular green alga, in its gastrodermal cells, rather than chlorophyll-containing chloroplasts. The photosynthetic activity of this alga provides oxygen to the hydra, while the metabolic activity of the former provides the carbon dioxide to the latter.

Pedal Disc

The pedal disc or basal disc is the proximal or aboral end of its body, which is closed and flattened. This is used to connect anything to a substratum for a short period. It has a glandular zone that secretes sticky chemicals for attachment, as well as a gas bubble that allows it to float.

Mouth and Tentacles

The hypostome is a conical elevation created at the distal or free opposite end of the body. At the apex, there is a round hole or mouth. A circlet of 6 to 10 slender, contractile, tubular thread-like structures, known as tentacles, is seen near the base of the hypostome (L. tentare, to feel). When the animal is hungry, it can expand these to several millimetres. Tentacles aid in the eating and movement of the animal.


In some individuals, the body of Hydra bears proximally lateral buds during various stages of development. A mature bud has its mouth, hypostome, and tentacles. It gives birth to a new individual upon dissociation.


The gonads are briefly arced by other structures that can be seen on the external surface. The oral end of the animal has conical testes, whereas the aboral end has rounder ovaries.


Hydra reproduces both asexually and sexually through budding and gamete production.


Asexual budding is the most common method of reproduction during the summer, when the animal is well-fed, healthy, and in a favourable environment. A bulge forms towards the base of the body as epidermal interstitial cells continue to divide. This develops as a bud, with an epidermal and gastrodermis wall, as well as an inner lumen that corresponds to the parent’s gastrovascular cavity. The bud grows in size, developing a mouth and tentacles at the free end. When the bud has reached complete maturity, it constricts at the base and eventually separates from the parent body. It consumes food and matures into an adult Hydra. Occasionally, many buds appear on a single parent at the same time, resembling a colonial hydroid.

Sexual Reproduction

Hydra reproduces sexually by fusing gametes, which happens in the fall. The interstitial cells of the epidermis generate gonads, which accumulate in the body wall to form bulges. Hydra is a hermaphrodite species, which means that spermatozoa and ova mature at distinct times, even though self-fertilization is avoided.


Testes are conical outgrowths of the body wall that can range in number from a few to a large number. The testis is created by cell division of interstitial cells of the epidermis and is protected from the outside by a capsule made of giant epidermal cells. Spermatogonia are interstitial cells at the base of the spermatogonia. They go through the normal spermatogenesis process and generate primary spermatocyte, secondary spermatocyte, and spermatid stages before becoming spermatozoa or sperms. A head and a lengthy vibratile tail characterise spermatozoa. Sperms are released when the testis wall ruptures at the apical nipple-like knob.


The ovaries take a little longer to grow than the testes. These ovoid formations are found around the body’s base. The growth of interstitial cells, which make up the main oogonia, also forms the ovary. However, after a while, one centrally positioned cell known as the Oocyte grows larger and amoeboid, with a huge nucleus. It ate its smaller surrounding interstitial cells, which turned into yolk or reserve food, which would be used up later while the young Hydra was still without a mouth to eat. As a result, the size of the Oocyte rises dramatically. It passes through two maturation divisions, producing two polar bodies and reducing the number of chromosomes to haploid. The mature egg, also known as the ovum, is a huge spherical mass packed with yolk granules. A single ovum is found in most ovaries, although two or more are detected in uncommon cases. When an egg is ready, the epidermis over it ruptures and withdraws, forming a cuplike receptacle that contains the exposed egg. The ovum is not immediately released but remains attached to the parent by a wide base.


When ripe sperm are released from testes, they swim around in the water until they come across an ovum and surround it. Although many sperms may enter the gelatinous coating, only one sperm enters the ovum and unites with it. Fertilization is the name given to this process, and the fertilised egg is referred to as a zygote.


Soon after fertilisation, development occurs

a) Cleavage: Because an egg has a little yolk, cleavage is ambiguous, total, and equal (holoblastic), resulting in blastomeres, which are equal-sized cells.

b) Blastulation: Cleavage produces a hollow spherical ball known as a blastula or coeloblastula. The blastocoel is the name for the middle narrow hollow. A single surface layer of blastomeres (equal-sized cells) is formed.

c) Gastrulation occurs when certain blastular wait cells separate and move inwards (multipolar ingression). Other cells divide tangentially to generate outer and inner cells (primary delamination). As a result, new cells fill the blastocoel, and the hollow blastula transforms into a solid gastrula. It is made up of a single layer of ectoderm-forming outside cells and an interior solid mass of endoderm-forming cells. As previously stated, multipolar ingression and primary delamination play a role in gastrulation and endoderm development in Hydra.

d) Encystation: In the centre solid mass of endodermal cells, a new cavity called a gastrocoel or archenleron forms. Meanwhile, around gastrula, ectodermal cells secrete a two-layered protective shell or cyst. The cyst wall has a thick, horny or chitinoid and spiky outer layer, and a thin gelatinous membrane on the inside. Different Hydra species can be distinguished by the pattern of their cysts. At this time, the encysted gastrula normally falls off the parent and sinks in the mud at the pond’s bottom or attaches to whatever solid object it comes into contact with its spikes. Gastrula is shed from the ovary before the formation of the shell in F. oligactis, and its sticky gelatinous covering binds it to aquatic plants. For several weeks, until next spring, the Encysted embryo is dormant and unchanged. It can survive drying and freezing conditions, as well as droughts and harsh winters. This resting stage is also likely to act as a dispersal stage, as it can be carried by wind or in mud on animals’ feet to nearby ponds with water.

e) Hatching: Development resumes with the arrival of favourable water and temperature conditions. Ectoderm produces interstitial cells, and mesogloea is secreted between two cellular layers. The embryo grows longer, and a circlet of tentacle buds appears at one end, with a mouth in the centre. The shell or cyst ruptures as the embryo becomes larger, and a juvenile Hydra with tentacles emerges. It quickly matures into an adult. In the development of Hydra, there is no free larval stage.

Top Questions

Question: Explain in simple words Hydra’s Immortality?

ANS: According to P. Brien (1955) and others, A Hydra is at least possibly immortal. There is a growing zone just below the tentacles, where interstitial cells give rise to all other body cells. Old cells are driven to the end of tentacles and pedal discs, where they are shed outside, in around 45 days. This process of cell replacement is never-ending. It has also been demonstrated that the Hydra only lives for a few days if the interstitial cells of the development zone are damaged.

Question: What do you understand by Budding in Hydra?

Ans: Budding is one of the most mysterious aspects of hydra development. Budding is the way Hydra reproduces asexually. Buds appear near the proximal end of the digestive zone, immediately above the peduncle, in actively growing Hydra. An increase in the optical density of the endoderm in the budding zone is the first indicator of bud development. The creation of a tiny conical protuberance follows, which grows in diameter and quickly elongates to form a tube. Tentacle rudiments form at the tube’s distal end, and a constriction appears at the tube’s proximal end at about the same time. A little full hydra eventually separates from the parent.

Question: What are the Different Phases of Bud individuation?

ANS: The process of bud individuation can be divided into three initiation, elongation, and regionalization phases of bud individuation.

Question: Explain the process of Regeneration in Hydra?

ANS: The ability of the hydra to regenerate lost body parts is one of its most interesting characteristics. When a hydra polyp is divided into two pieces, the headpiece regenerates the missing foot, while the foot regenerates the lost head. In hydra, this process does not necessitate growth (an increase in cell counts), at least in the early stages; it is hence called’morphallaxis,’ as a contrast to epimorphosis, which occurs in amphibian limb and tail regeneration and necessitates growth. As a result, the regenerated polyp is smaller than the original. Except for the tentacles and the basal disc, almost every portion of the hydra’s body is capable of regeneration to some degree.

 A hydra cell pellet can regenerate into a polyp. When a hydra is cut into three pieces, the central portion, which is missing both the head and the foot, regenerates a new head and foot on the sides where the original head and foot were. This implies that information exists in the cells of the centre component that directs the regeneration of lost portions in the original orientation.

Question: Explain the Lewis Wolpert theory of morphogen gradients?

ANS: Lewis Wolpert proposed the theory of morphogen gradients and positional information in the late 1960s based on hydra research, as well as limb regeneration experiments in the chick embryo. Morphogens are chemical substances that cause morphogenesis to occur (development of form and shape). In a nutshell, Wolpert argued that information in the regenerating middle component, in the form of chemical concentration gradients, drives cell destiny towards one (head) or the other (foot) pathway. In hydra, for example, there is a gradient in head-forming molecule concentration, with the highest concentration at the head end and the lowest concentration at the foot end. As a result, the concentration of the head-forming molecule in any piece of the body column will always be highest at the cut end closest to the original head. The foot-forming molecule would behave similarly but in the other direction. As a result, the missing buildings are regenerated at appropriate locations. Wolpert further postulated that cells in a gradient can sense their position in the gradient based on the concertation of the morphogen they come into contact with (or are exposed to). Morphogen gradients have since been discovered to be critical to several developmental processes in all animals. Hydra’s remarkable regenerating ability allows researchers to investigate pattern formation mechanisms at the morphological, cellular, and molecular levels. Deciphering the mechanics of hydra regeneration can give us insights into features of regeneration that may be beneficial in understanding why more complex animals have a considerably reduced capacity to replace destroyed tissues and organs. Understanding hydra regeneration may one day help us figure out how to boost the regenerative potential of other organisms.

Question: Is Regeneration a form of Reproduction?

ANS: An individual’s ability to regenerate lost or injured body parts is referred to as regeneration. If a living Hydra animal is sliced into two, three, or more pieces, each missing piece grows and the animal becomes entire. A piece of Hydra with a diameter of 1/6 mm or larger can regenerate a full person. Because regeneration is not a typical form of multiplication, it should not be considered reproduction. The remarkable generating abilities of totipotent interstitial cells allow hydras to regenerate. Hydra’s regenerating component keeps polarity, which is one of its distinguishing characteristics. The end closest to the mouth grows a mouth and tentacles, while the end closest to the base grows a new pedal disc. If the Hydras are of the same species, parts of one can simply be grafted onto another. Grafting can be done in a variety of ways, resulting in strange outcomes. Trembley discovered that splitting the head end of a Hydra in two and slightly separating the sections results in a Y-shaped specimen, or “two-headed” person. Trembley was able to create a “seven-headed” Hydra by further dividing heads.

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