| Hydra | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Scientific classification | ||||||||||||
|
||||||||||||
| Species | ||||||||||||
|
Hydra americana |
Hydra is a genus of simple, fresh-water animals possessing radial symmetry. Cnidaria (naɪˈdɛəriə is a phylum containing some 9000 Species of Animals found exclusively in aquatic mostly marine, environments Hydrozoa ( hydrozoans) are a Taxonomic class of very massive predatory animals which can be solitary or colonial and which mostly live in saltwater Hydroida is an obsolete Cnidarian order which united such animals as hydras, Hydromedusae, and many marine attached hydroids. Carl Linnaeus (Latinized as Carolus Linnaeus, also known after his ennoblement as, May 23 new style (13 May old style 1707 who laid the foundations for In Biology, a species is one of the basic units of Biological classification and a Taxonomic rank. Hydra oligactis, also known as the brown Hydra, is a species of hydra found widely dispersed in the northern Temperate zone. Hydra viridis (also known as Chlorohydra viridis) is a species of ''Hydra'' found widely dispersed in the northern temperate zone Hydra viridissima also known as the green hydra, is a cosmopolitan species of hydra found in both temperate and tropical fresh waters A genus (plural genera from Γένος Latin genus "descent family type gender" is a low-level Taxonomic "Bilateral symmetry" redirects here For bilateral symmetry in mathematics see Reflection symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. Cnidaria (naɪˈdɛəriə is a phylum containing some 9000 Species of Animals found exclusively in aquatic mostly marine, environments Hydrozoa ( hydrozoans) are a Taxonomic class of very massive predatory animals which can be solitary or colonial and which mostly live in saltwater They can be found in most unpolluted freshwater ponds, lakes and streams in the temperate and tropical regions by gently sweeping a collecting net through weedy areas. They are usually a few millimeters long and are best studied with a microscope. A microscope ( Greek: ( micron) = small + ( skopein) = to look or see is an instrument for viewing objects that are Biologists are especially interested in hydras due to their regenerative ability. In Biology, an Organism is said to regenerate a lost or damaged part if the part regrows so that the original function is restored Hydras appear to be unique among animals in that they do not undergo senescence (aging). Senescence refers to the biological processes of a living Organism approaching an advanced age (i
Hydra is a small animal with a body length ranging from 1 mm to 20 mm when fully extended. To help compare different Orders of magnitude this page lists Lengths between 10&minus3 m and 10&minus2 m (1 mm and 1 cm It has a tubular body secured by a simple adhesive foot called the basal disc. Gland cells in the basal disc secrete a sticky fluid that allows for its adhesive properties. At the free end of the body is a mouth opening surrounded by one to twelve thin, mobile tentacles. Tentacles can refer to the elongated flexible organs that are present in some animals especially Invertebrates and sometimes to the hairs of the leaves of some insectivorous Each tentacle, or cnida (plural: cnidae), is clothed with highly specialised stinging cells called cnidocytes. A cnidocyte, cnidoblast or nematocyte, is a type of Venomous cell unique to the Phylum Cnidaria ( Corals A cnidocyte, cnidoblast or nematocyte, is a type of Venomous cell unique to the Phylum Cnidaria ( Corals Cnidocytes contain specialized structures called nematocysts which look like miniature light bulbs with a coiled thread inside. A cnidocyte, cnidoblast or nematocyte, is a type of Venomous cell unique to the Phylum Cnidaria ( Corals At the narrow outer edge of the cnidocyte is a short trigger hair. Upon contact with prey, the contents of the nematocyst are explosively discharged, firing a dart-like thread containing neurotoxins into whatever triggered the release. A neurotoxin is a Toxin that acts specifically on nerve cells ( Neurons, usually by interacting with Membrane proteins such as Ion channels To humans, this poses a nuisance at worst; however, to some prey, this strike can be paralyzing.
Hydras mainly feed on small aquatic invertebrates such as Daphnia and Cyclops. Daphnia are small Planktonic Crustaceans between.2 and 5 mm in length This article is about the crustacean See Cyclops (disambiguation for other meanings All species of Hydra exist in a mutual relationship with various types of unicellular algae. Mutualism is a Biological interaction between individuals of two different Species, where both individuals derive a fitness benefit for example increased Algae ( sing. alga are a large and diverse group of simple typically Autotrophic organisms ranging from Unicellular to Multicellular forms The Hydra affords the algae protection from predators and in return, photosynthetic products from the algae are beneficial as a food source to the Hydra . Photosynthesis is a Metabolic pathway that converts Light Energy into Chemical energy.
Contents |
Morphology
Hydra have two main body layers separated by mesoglea, a gel-like substance. Mesoglea, also known as ectoplasma, is the clear inert jellylike substance that makes up most of the bodies of Jellyfish, Comb jellies and certain primitive The outer layer is the epidermis and the inner layer is called the gastrodermis. In Anatomy, squamous epithelium (from Latin squama, "scale" is an Epithelium characterised by its most superficial layer consisting The gastrodermis is the inner layer of cells that lines a Gastrovascular cavity of Cnidarians. The cells making up these two body layers are relatively simple.
The nervous system of the Hydra is a nerve net, which is structurally simple compared to mammalian nervous systems. For the album by Brian Eno, see Nerve Net (album. A nerve net is a type of simple Nervous system that is found in members of the phylum Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands Hydra does not have a recognizable brain or true muscles. The brain is the center of the Nervous system in animals All Vertebrates and the majority of Invertebrates have a brain Muscle (from Latin musculus, diminutive of mus "mouse" is contractile tissue of the body and is derived from the Nerve nets connect sensory photoreceptors and touch-sensitive nerve cells located in the body wall and tentacles. Photoreceptor can refer to In anatomy/cell biology Photoreceptor cell: a photosensitive cell most commonly referring to a specialized type of neuron
Respiration occurs by diffusion through the epidermis. Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement Some excretion and transportation also occurs in this manner.
Many members of the Hydrozoa go through a body change from a polyp to an adult form called a medusa. Hydrozoa ( hydrozoans) are a Taxonomic class of very massive predatory animals which can be solitary or colonial and which mostly live in saltwater In Zoology, a polyp is one of two forms of individuals found in many species of Cnidarians The two are the polyp or hydroid and the medusa. In Biology, a medusa (plural medusae is a form of Cnidarian in which the body is shortened on its principal axis and broadened sometimes greatly in contrast However, all hydras remain as a polyp throughout their lives.
19th century biologists reported that the Hydra was such a simple animal that it was possible to force one through gauze to separate it into individual cells; if the cells were then left to themselves, they would regroup to form a hydra again.
Motion and locomotion
If a Hydra is alarmed or attacked, the tentacles can be retracted to small buds and the body column itself can be retracted to a small gelatinous sphere. Hydras generally react in the same way, regardless of the direction of the stimulus and this may be due to the simplicity of the nerve net,
.JPG/250px-Hydras_(8).JPG)
Hydras are generally sedentary or sessile, but do occasionally move quite readily, especially when hunting. Sessile is a term in Biology with two distinct meanings In botany and medicine In Botany, sessile means "without a stalk They do this by bending over and attaching themselves to the substrate with mouth and tentacles and then release the foot, which provides the usual attachment. The body then bends over and makes a new place of attachment with the foot. By this process of "inch-worming" or "somersaulting," a hydra can move several inches (c. 100 mm) in a day. Hydras may also move by amoeboid motion of their bases, or by simply detaching from the substrate and floating away in the current. Amoeboids are Unicellular lifeforms that mainly consist of Contractile vacuoles, a nucleus, and cytoplasm as their basic structure
Reproduction
When food is plentiful, many hydras reproduce asexually by producing buds in the body wall which grow to be miniature adults and simply break away when they are mature. Asexual reproduction is a form of reproduction which does not involve Meiosis, Ploidy reduction or Fertilization. Budding (also called burgeoning) is the formation of a new Organism by the protrusion of part of another organism When conditions are harsh, often before winter or in poor feeding conditions, sexual reproduction occurs in some hydras. The Evolution of sexual reproduction is a major puzzle The first Fossilized evidence of sexually reproducing Organisms is from Eukaryotes of the Stenian Swellings in the body wall develop into either a simple ovary or testes. The testes release free swimming gametes into the water and these can fertilise the egg in the ovary of another individual. The fertilized eggs secrete a tough outer coating and, as the adult dies, these resting eggs fall to the bottom of the lake or pond to await better conditions, whereupon they hatch into miniature adults. Hydras are hermaphrodites and may produce both testes and an ovary at the same time. A hermaphrodite is an organism having both male and female reproductive organs
Feeding
When feeding, hydras extend their body to maximum length and then slowly extend their tentacles. Despite their simple construction, the tentacles of hydras are extraordinarily extensible and can be four to five times the length of the body. Once fully extended, the tentacles are slowly manoeuvred around waiting for contact with a suitable prey animal. Upon contact, nematocysts on the tentacle fire into the prey and the tentacle itself coils around the prey. A cnidocyte, cnidoblast or nematocyte, is a type of Venomous cell unique to the Phylum Cnidaria ( Corals Within 30 seconds most of the remaining tentacles will have already joined in the attack to subdue the struggling prey. Within two minutes, the tentacles will have surrounded the prey and moved it into the opened mouth aperture. Within ten minutes, the prey will have been enclosed within the body cavity and digestion will have started. The hydra is able to stretch its body wall considerably in order to digest prey more than twice its size. After two or three days, the indigestible remains of the prey will be discharged by contractions through the mouth aperture.
The feeding behaviour of the hydra demonstrates the sophistication of what appears to be a simple nervous system.
Morphallaxis
The hydra undergoes morphallaxis (tissue regeneration) when injured or severed. See the morphallaxis article for more details. Morphallaxis is the regeneration of specific tissue in a variety of Organisms due to loss or death of the existing tissue
Senescence
It has often been assumed that hydras are unique among animals in that they do not undergo senescence (aging), and so are biologically immortal. Senescence refers to the biological processes of a living Organism approaching an advanced age (i Biological Immortality can be defined as the absence of a sustained increase in rate of mortality as a function of chronological age Evidence for this was provided by Martinez (1998). [1]
References
- ^ Martinez, D. E. (1998) "Mortality patterns suggest lack of senescence in hydra. " Experimental Gerontology 1998 May;33(3):217-225. Full text.
- Gilberson, Lance, Zoology Lab Manual, 4th edition. Primis Custom Publishing. 1999
- Solomon, E. , Berg, l. , Martin, D. , Biology 6th edition. Brooks/Cole Publishing. 2002
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic