Lab 6 : Cnidarians

Bygliding on its foot, by inching along with its tentacles and foot, by somersaulting from foot, to tentacles,to foot, or by floating on a bubble of gas secreted by the foot.

The conical elevation with the mouth opening at its tip. The mouth serves as both theentrance (mouth) and exit (anus) to the gastrovascular cavity. Often times the actual opening of themouth is difficult to see. When Hydra are not actively feeding, cells stick together and cover theopening. Also, the opening is visible only if your slide is a true longitudinal section through themidline of Hydra. If you do see the mouth, share the slide with others in your group.

Food catching arms that radiate from the hypostome. The tentacles contain specializedstinging cells (cnidocytes) used in prey capture and defense.

Location where digestion and absorption of food occurs. Some gas exchange(respiration) also occurs here in the cells lining the cavity (gastrodermis) as well as across theepidermis.

Composed of mesoglea sandwiched between two tissue layers of cells:• Epidermis: outer layer of epithelial cells. Composed of different cell types (see next section).• Gastrodermis: inner layer of epithelial cells. Composed of different cell types (see next section).

A thin, gelatinous layer between the epidermis and gastrodermis. In Hydra it is visible onlyas a thin dark line. However, this layer is very thick in medusa forms of cnidarians. It provides supportas a hydrostatic skeleton.

These cells act both as epithelial cells by covering and lining both surfacesand as muscle cells by having contractile muscle fibers. One type of these cells has longitudinalmuscle fibers and is located in the epidermis. The other has circular muscle fibers and is located in thegastrodermis. These two cell types work antagonistically to enable movement in the Hydra. In thegastrodermis, epitheliomuscular cells are also able to engulf partially digested food particles from thegastrovascular cavity and complete digestion intracellularly.

Located in the epidermis and most concentrated in the tentacles andhypostome. These cells contain the nematocyst capsules, which are triggered by cnidocils,bristle-like projections on the outer surface of the cnidocyte. The cnidocils are modified epidermalcilia. The nematocysts of cnidarians can be of various types: stinging, entangling, and adhesive.Nematocyst types are important in the classification of some cnidarians.

Nerve cells that extend along the base of the epidermis and the gastrodermis. These neuronsform a nerve net in each tissue layer. Neurons running through the mesoglea connect the two nervenets together.• Sensory cells: a type of neuron located in the epidermis. These receptor cells are sensitive toexternal stimuli such as touch (pressure) or chemicals in the water. Impulses from sensory cellsare relayed to neurons of the nerve net that synapse with epitheliomuscular cells. The result iscoordinated muscular movement.

No quresion

Located in the epidermis of the basal disc. They secrete a sticky substance used inanchoring the basal disc to the substrate. There are also gland cells in the gastrodermis, which secretedigestive enzymes.

Located between the epitheliomuscular cells of the epidermis. Because of theirsmall size and placement, they may be difficult to locate. These cells are totipotent, meaning that theyact like stem cells by being able to divide continuously and to differentiate into other cell types suchas neurons and cnidocytes.

The sperm and eggs are produced in gonads that are visible as small mounds in the epidermisof the body column. Sperm is produced in testes on the upper half of the body, near the tentacles, andone egg is produced in each ovary in the lower half of the body, near the foot. The ovaries areslightly larger than the testes. Some individuals possess the capability of producing both types ofgonads and gametes and are therefore called hermaphrodites. However, most hydra have separatesexes so ovaries and testes are found on different individuals.Sexual reproduction in Hydra usuallyoccurs in the fall. During this time, spermare released into the water and reach an eggby entering the ovary through a hole in theepidermis. The resulting zygote forms ahard covering and drops to the sediments.The zygote remains dormant until thearrival of spring when a new polypemerges.􀀂 Often the

By the budding of a new individual from the body of an adult by theprocess of mitosis. This new Hydra is genetically identical (a clone) to the adult. Eventually the buddetaches itself from the parent Hydra when it is large enough to capture its own food and liveindependently. This process takes about 2-4 days.

Gonionemus is a small medusa reaching about 2.5 cm in diameter. It inhabits shallow temperate marinewaters, and some species of this genus spend most of their life clinging to seagrasses rather thanswimming. Some members of this genus are poisonous to humans due to a powerful toxin in theirnematocysts.Unlike most cnidarians in the class Hydrozoa, Gonionemus spends most of its life in the medusa stage.The medusa stage of a cnidarian resembles that of an inverted polyp with the mouth and tentacles facingdownwards. Much of the body is expanded to form a dome-like structure with a very thick mesoglea thatprobably helps with buoyancy and acts as a hydrostatic skeleton.