Belonging to the phylum Coelenterata, the class Hydrozoa is best known for the polyp state, which is revealed, under microscope examination, to form extensive and beautiful colonies. The multiply stained specimen illustrated below is a member of the Pennaria genus, and was captured in brightfield illumination on the MIC-D digital microscope.

Pennaria feature tentacles that occur in two forms: filiform and capitate. The filiform tentacles resemble those of hydra, and are long and tapering, with batteries of nematocysts distributed over the entire surface. In contrast, capitate tentacles occur with their entire nematocyst array confined to a single knob on the tentacle end. In Pennaria, medusa buds (even very large examples) lack tentacles, and the gonads develop in the epidermis of the manubrium.