Biology: Hydra Characteristics

Body Structure

• Hydra is a freshwater solitary animal with a cylindrical body (2-20 mm in length)
• It has a basal disc for attachment and a free oral cone (hypostome) bearing the mouth
• The mouth is situated on a conical hypostome encircled by 6-10 hollow filiform tentacles in one cycle
• The tentacles are provided with nematocysts

Reproductive System

• Gonads appear as buds on the side of the body stalk
• Several testes arise on the sides of the upper third of the body near the oral end
• One ovary is present near the basal disc
• Lateral buds may be present by asexual reproduction

Nematocysts

• There are four kinds of nematocysts in Hydra: penetrants, volvents, small stereoline glutinants, and large streptoline glutinants
• Each type of nematocyst has a specific function: defense, catching prey, and food collection
• Nematocysts respond only to mechanical stimuli

Body Wall

• The body wall of Hydra viridis is diploblastic, consisting of an outer ectoderm (epidermis) and inner endoderm
• The ectoderm and endoderm are separated by a thin structureless jelly-like mesogloea
• The body wall encloses a digestive cavity (coelenteron) that extends into the tentacles

Ectoderm and Endoderm

• Ectoderm cells are musculo-epithelial, interstitial, nematoblast, sensory, nerve, mucous cells, and germ cells
• Endoderm cells are musculo-nutritive, interstitial, sensory, nerve (less developed cells), glandular or secretory cells, and few mucous cells around the mouth
• Musculo-epithelial cells have longitudinal muscle fibres that contract to shorten or bend the body
• Musculo-nutritive cells have circular muscle fibres that contract to narrow and elongate the body

Nutrition and Digestion

• Hydra is carnivorous, feeding on freshwater microorganisms, small crustaceans, and insect larvae
• Food is digested extracellularly by secreting proteolytic enzymes from the glandular cells of the endodermal layer
• Nutritive muscle cells engulf small fragments of food, forming food vacuoles and undergoing intracellular digestion
• The products of digestion are absorbed by the nutritive cells and circulated by cellular diffusion

Respiration and Excretion

• Respiration and excretion take place by diffusion

Locomotion

• Hydra is essentially sessile and lives attached to the substratum by a sticky secretion
• It can move from one place to another by gliding, walking, somersaulting, and floating
• Walking is the most common method of locomotion, using the adhesive threads of the small glutinant nematocysts