Fresh water polyp, known as hydra, is chosen as a type of the two layer metazoan animals belonging to the phylum coelen terata o cnidaria which is characterised by the presence of the following distinguishing features; [1] There is a single central cavity , the coelen teron surounded by the two layered body walll and having a single opening to the exterior.[2]They have nematoblasts or stinging cells.[3]They are acoelomates,i.e., the true coelum,whicjh is the cavity between the gut wall and the body wall, is absent..[4]The body is radially symmetrical,ie.,the planes of symetry radiates from central point like the spokes of the wheel; any plane pa ssing thruogh the centre would divide the animal into two similar parts,each being the mirror image of the other. [5]There are a numbero of atentacles which are usually thread l8ike out growths of the body and bear battweries of stinging cells on them. HABITAT AND RABBIT Hydra is a fresh water polyp, which remains attacherd to debris or submerged acquatic weeds.In looking for those animals it is best to feel a grass jar with aquatic weeds and to leave it undisturbed for some time in some lighted place.Hydra,if present,will be found attached to the side or the bottom of the glass jar.Usually they are near the surface of the water where more of oxygen and lights are available.Ifattached to the bottom of the glass jar the body is usually in an opright position, if they are on the side of the jar, the body is in most cases horizontol and if they are attaached to the surface film, the body is allowed to hang downward.Hydra is exclusively carnivorous, feeding on water -fleas,insect larvae and eggs and fingerlings of fishes. DIFFERENT SPECIES OF HYDRA There are several species of the hydra and the commonly known species include hydra viridis, h. gangeticus, h.vulgaris and h. viridis ,which is bright green and active animal,has shorty tentacles. its colour is due to the presence of a green alga, chlorella vulgaris. it is now known as chloro hysdra virridissima. The colour of H.fusca is greenish due to the presence of zooxan thaellae H.vulgaris is more or less colour less. of all these speciesH .oligactis is very commonly found throughout india.EXPERIMENT.To observe the appearence of the hydra. place two twop shorts pieces of hydra shoots parallel and about inch apart in a drop of water on a glass side. with a drooper transfer a hydra to the water.Cover it with a cover glass and supported by the water weed .Note the appearence of t he hydraunder hand lens or low power the microscope.Note structure and sketch. EXTERNAL FEATURES The body of hydra resembles a narrow elastic tube which is closed at one wend and open at other . The closed end by which hydra remains attached to some object is known as thwe foot or a basal disc and is refered as the aboral end of the animal. The foot secrets a sticky substances which not only anchor the animal, but also serves as a locomotry organ. At the other end of the body there is a conical process hypostomwe or the oral oral cone in the centre of which is situated a somewhat irregular star- shaped mouth.During rest of digestion when the mouth is closed it ias very small pore but while shallowing the prey, the mouth and the surronding hypostome can dilate to enorrmous size. Surrounding the mouth there are sleder contactiles and hollow tentacles, which vary in number in different species but are seldom less than sis and more than ten. The body , when fully exztended, may attain a length of 20 mm or more;when its diameter gets reduced in the proportion, its length increases.The tentascle being very contractile, are cappable of remarkable extension; they may streched out from small blunt projection to exremely thin threads, 7 cm or more in length ,and is fully extended condition they may be so very thin and transparent as to be barely visible even with the hand lens.The long armed polyp H .oligactis can extend its tentacles toa length of eight inches. when food supply is abudant and the water is warm, specimens of hydra are frequently found having attached to them near the proximal end one or more small hydras differing from thier parents only in size or in the incomplete develop ment of the tentacles.These are kinown as buds these buds are lateral hollow out growths of the body cylinder. Seasonally there may be objects rounded projection on the body, whjich are the sexual organs, the testes and the ovaries are knob like and are more towards the base. STRUCTURE OF HYDRA Asection of hydra shows that the body contain tthe single central cavity, communicating at tyhe mouth with the exterior and extendinmg into the tentacles as well. the food is taken into this cavity and here it undergoes digestion, therefore , this cavity is known as the coe len teron or gastro vascular cavity .The body wall is made of two layers of cells , seperated by a non cellular gelantinous layer kown as meso gloea. Of the the two layeras of cell, the outyer one forms acovering to the whole body, inmcluding the basal disc the tentacles and the hypostome and is called the epiderims At the mouth it becomes continuos with the inner celll- layer or gastrodermis which forms lining to the coelen teron or gastro vascular cavity and extends up into the tentacles to form their lining .The cells of epiderims , which comes in contact with the changing conditions of the animal environment are, trherefore , mainly protyective and sensory . The cells of the gastro dermis on the other hand , comes in contact with food material and are , there fore, mainly secretory and digestive. Animals built upon this planh , i.e., with only two fundamental cellular l ayers are described as diplo blastic. The epidermisd and gastro dermis are both composed for the most part, of columnar cells, which cvannot said to form typical tissues for in each layer there are several kinds of cells adapted to carry on different functions.
No comments:
Post a Comment