Hydras are Polyp expressing Cnidarians that are interesting in phenotype and diversity. They express a similar structure underneath a dissection scope as they do under high-powered, slide-based microscopes. It is incredible to look at preserved samples in slides, as one may readily observe the abundance of endosymbiotic algae or Zooxanthellae. Fresh water hydrozoans will express different types and abundances of these mutualistic algae than their oceanic relatives. Specimens of Hydra viridis have been known to house in their relatively small bodies, up to 1.5 x 105 endosymbiotic algae (Pardy, R. 1974)1. Other notable structures are the Cnidocytes that are housed within the tentacles; pressurized in visible pustules, covering much of the surface area that encompasses each tentacle.
There are two different general forms that are used to describe the morphology of Cnidarians. They are the polyp form, as mentioned above and the medusa form, that is most notably observed in organisms such as jellies. Polyps are distinguishable in their abilities of retraction and extension; having a body stalk with a tubular shape, complete with a single orifice and multiple tentacle appendages. Medusa forms often contain a bulb-body structure, with either trailing or superior facing tentacles, digestive tubes and other appendages (see illustration). Many Cnidarians have the ability to switch between these morphologies over the course of their life.
Hydrozoans may also be physiologically and neurologically connected. This is often expressed in certain species for the betterment of the whole species. The benefits of being an individual within a colony is the aspects of protection, resource sharing and even passing of signal impulse. If one individual is stimulated by a threat the signal will be sent throughout the whole of the colony, initiating a retraction across the whole of the colony.
When analyzing skeleton corals there are distinct factors with which to pay special attention. CaCO3 deposits are constantly accumulated around the living organisms throughout the course of their life-cycles. The organism is then housed within these Calcified deposits, and uses this waste product to its advantage as protection. Analyzing the ‘skeletons’ left behind after the death of these organisms lends light to how they lived, developed and evolved over time. All of the perforations that might be observed were once home to a polyp that made up the actual live being of a coral.
ROSEVELT L. PARDY, “SOME FACTORS AFFECTING THE GROWTH AND DISTRIBUTION OF THE ALGAL ENDOSYMBIONTS OF HYDRA VIRIDIS,” The Biological Bulletin 147, no. 1 (August 1974): 105-118.