Ward's World Working with Guides

Page 2 Hydra are freshwater coelenterates found throughout the world. They range in size from less than 1 mm to 5 mm. Their simple structure consists of a polyp — a slender stalk with a row of tentacles surrounding the mouth at the top and a pedal disc at the base, by which the Hydra attach themselves to substrate in streams and ponds. Voracious predators, Hydra's tentacles are armed with stinging cells (nematocysts) that "harpoon"their prey (usually small crustaceans). The tentacle can then rapidly retract to draw the food into the mouth. Common species include Hydra vulgaris (brown Hydra) and Chlorohydra viridissima (green Hydra). Green Hydra differ from brown Hydra in that their green color is caused by their symbiont, an alga (Chlorella) Green Hydra are also smaller than brown Hydra, ranging from about 0.5 to 2 cm. Tissue Function Hydra's two main cell layers, the epidermis (outermost cell layer) and gastrodermis (inner cell layer) are separated by a thin mesogleal plate, an "acellular"substance. These cell layers are incipient tissues, consisting chiefly of one cell type but containing a number of other cells; they have broad functions rather than the narrow functions typical of true tissues. The epidermis consists of vacuolated musculo-epithelial cells containing muscle threads (myonemes). These muscle threads attach to the mesogleal plate; when they are contracted, the Hydra's body shortens. Epidermis gland cells on the pedal disc form adhesive secretions. Small interstitial cells are packed in at the base of the musculo-epithelial cells; they are especially prominent in the "growth zone" of the anterior part of the column region and at points where buds or sex organs develop. These cells are unspecialized, and can replace cells of any other type. Many interstitial cells become nematocysts (stinging cells). Large nutritive-muscular cells in the gastrodermis feature transverse myonemes that lengthen the body when they contract. The gastrodermis is specialized by region, as shown by the distribution of cells and the changes in nutritive-muscular cell form. Mucous gland cells are abundant near the Hydra's mouth, and enzymatic gland cells secrete enzymes for extracellular food digestion. The column contains gastrodermis where digestion and absorption occurs; the stalk region has a low, inactive gastrodermis. Reproduction Hydra usually reproduce asexually, by budding. This process lasts two to four days, ending with the daughter Hydra detaching from the parent to become a separate organism. Hydra can also reproduce sexually, although this is much less common. Differentiation must be induced by environmental factors such as high carbon dioxide levels or a change in temperature; sexual reproduction usually occurs in the autumn. Different species of Hydra are either dioecious or hermaphroditic. Testes form from interstitial cells that produce a swelling in the upper third of the body. Spermatogenesis results in flagellated spermato- zoa. In the ovary, a large, yolk-filled oocyte forms in a manner similar to the testes. After the epidermis ruptures over the mature ovum, it is fertilized by spermatozoa released into the surrounding water. The fertilized ovum remains on the parent's body, where it under-goes cleavage and gastrulation to become an embryo. A protective shell, or theca, forms around the embryo, after which it detaches from the parent. The egg may remain dormant for some time before the embryo emerges as a small immature polyp with a mouth, body, and tentacles. Working with Hydra Introduction

• Cover