Hydra

            The Hydra is an excellent organism for the study of the polyp form of the Phylum Cnidaria.  Although a very simple organism, it illustrates a definite level or organization into tissues. 

Hydra sp.

Examine slide #59 of stained cross sections through the body wall of Hydra.  Note that the body wall surrounding the gastrovascular cavity consists of two distinct layers of cells:  an outer layer called the epidermis and an inner layer called the gastrodermis.  These two layers are held together by a thin, non-cellular layer, the mesoglea (Figure 2). 

 

Figure 2:  Diagram of the cross-section through the body wall of Hydra sp. (top) and the microscopic view of Hydra (bottom).

Place a living Hydra in a small dish of water and examine with the dissecting microscope.  At the upper end of the animal, observe several elongate, actively moving tentacles, which are used to capture food (Figure 3).  At the centre of this circle of tentacles, is the mouth.  Tap the edge of your dish and observe the extreme contractility of this organism.

            To observe your specimen ingesting food, add several Daphnia (or other available food organisms) to your dish and observe with the dissecting microscope. When the tentacles of the Hydra come into contact with its prey, the prey jerk for several minutes before becoming still.  The food organism reacts in this way because it is being stung by numerous wart-like nematocysts on the tentacles.  Once the food organism has been ingested, it will be degraded within the gastrovascular cavity and the small particles engulfed by the gastrodermal cells by phagocytosis. 

Being small, exclusively aquatic organisms cnidarians require neither a special respiratory nor circulatory system.  The entire body surface can participate in gas exchange since all cells in these diploblastic (two-layered) animals are constantly moist and exposed to the gas-containing medium. In addition, no cell is ever far enough from the source of either nutrient materials or gases to necessitate any more elaborate transport than either direct diffusion from cell to cell or diffusion through the very watery mesoglea that exists between the two cell layers (the epidermis and the gastrodermis).

Hydra reproduce asexually by simple budding and sexually by the production of sperm and ova.  Budding consists of a simple outgrowth from the body wall.  The gastrovascular cavity of the bud is initially continuous with that of the parent.  The bud will separate from the parent at maturity.

Examine slide # 58 observe Hydra budding (Figure 3). 

The gametes for sexual reproduction are produced in organs called spermaries (testes) and ovaries, which are protuberances along the body wall (Figure 3).  Both sex organs may be present in a single organism or in two separate organisms.

Examine slides #60 and #61 and try to locate these gamete-producing organs.

Figure 3:  Longitudinal section through Hydra.