11th Class Biology Animal Kingdom Some Representative Animals

(1) Hydra belongs to class Hydrozoa of phylum coelenterata.

(2) Trembly (1744), a Swiss biologist discovered Hydra.

Linnaeus (1758) gave the name Hydra, a Greek word, means ‘Water serpent’ based on its ability to regenerate its lost parts.

(3) Hydra is a solitary polyp found in freshwater (stagnant). Among coelenterates Hydra is one of the smallest polyps.

(4) It is colourless carnivorous coelenterate having radial symmetry.

(5) Hydra is a diploblastic and has tissue grade of organization with division of labour on morphological basis.

(6) Chlorohydra viridissima is called green hydra. It is green because of symbiotic association with a unicellular green algae Chlorella vulgaris. Algae live in the musculonutritive cells of Hydra.

(7) Hydra has a cylindrical body with 6-10 hollow tentacles. It help in locomotion and food capture, so analogous (correspond functionally) to pseudopodia of Amoeba.

(8) Mouth is situated on a manubrium or hypostome. It is the most sensitive in the body. Hydra has no anus.

(9) The body wall of Hydra consists of ectoderm and endoderm, in between a thin, delicate, transparent and non-cellular mesogloea.

(10) Ectoderm consists of epithelio-muscular cells, sensory cells, nerve cells, interstitial cells (totipotent) and stinging cells or cnidocytes having nematocysts.

(11) Inner gastrodermis has nutritive muscular cells, gland cells, nerve cells, sensory and interstitial cells. Nutritive muscular cells bear both flagella and pseudopodia.

(12) The contraction of muscle fibres in endothelio-muscular cells or nutritive muscle cells reduces the diameter of the body and works like circular muscles.

(13) Mesogloea is thin and acellular consisting of a proteinaceous matrix and it can be crossed by interstitial cells. It is neither cellular nor fibrous.

(14) Cnidoblasts or nematocysts are derived from interstitial cells of epidermis.

(15) Body cavity of Hydra is called coelenteron or gastrovascular cavity. Coelenteron serves the double purpose of digestion and circulation.

(16) Nematocysts are found only in epidermis mainly on tentacles. Nematocysts are also known as “independent effectors”.

(17) Hydra paralyses its prey by nematocyst. If all nematocysts of a Hydra are removed it would affect its capacity to capture prey.

(18) Nematocyst plays an important role in locomotion, food capture both offence and defence.

(19) Hydra has four types of nematocysts : Penetrants or stenoteles (largest), valvents (smallest), stereoline glutinants (small, atrichous) and streptoline glutinants (large holotrichous)

(20) Digestion in Hydra is first extracellular (in gastrovascular cavity) and then intracellular (in endoderm cells).

(21) Hydra has no specialized cells for respiration, it respires by means of general body surface.

(22) Nitrogenous excretory product in Hydra is ammonia and it is removed through general body surface.

(23) Hydra possesses a very primitive nervous system consisting of a synaptic network of bipolar and multipolar nerve cells, but brain is absent.

(24) Hydra is monoecious or dioecious. Most species are dioecious or unisexual. Bisexual species of Hydra are protandrous, so avoid self-fertilization.

(25) Hydra reproduces asexually by exogenous budding, a type of vegetative propagation, and sexually by formation of gametes. Hydra reproduces by budding when plenty of food is available.

(26) Hydra normally possesses a single ovary (in aboral region) and many testes (in oral region).

(27) Fertilization occurs externally on the body by the entry of sperm into ovum.

(28) The developing embryo in Hydra drops down from the body of parent after the formation of gastrula.

(29) In the development of Hydra there is no moulting or ecdysis.

(30) No free larval stage (only a planula like stage) occurs in Hydra.

(31) Hydra has great regeneration capacities. A piece of Hydra will regenerate into a full Hydra if it contains a part of epidermis and gastrodermis and size is not less than 1/6 mm in diameter.

Common Names