[2] Hydra has a tubular, radially symmetric body up to 10 mm (0.39 in) long when extended, secured by a simple adhesive foot known as the basal disc.

Gland cells in the basal disc secrete a sticky fluid that accounts for its adhesive properties.

At the free end of the body is a mouth opening surrounded by one to twelve thin, mobile tentacles.

Cnidocytes contain specialized structures called nematocysts, which look like miniature light bulbs with a coiled thread inside.

Upon contact with prey, the contents of the nematocyst are explosively discharged, firing a dart-like thread containing neurotoxins into whatever triggered the release.

[7] Respiration and excretion occur by diffusion throughout the surface of the epidermis, while larger excreta are discharged through the mouth.

Once fully extended, the tentacles are slowly maneuvered around waiting for contact with a suitable prey animal.

[16] The feeding response in Hydra is induced by glutathione (specifically in the reduced state as GSH) released from damaged tissue of injured prey.

[18] Other methods rely on counting the number of Hydra among a small population showing the feeding response after addition of glutathione.

[20] In this method, the linear two-dimensional distance between the tip of the tentacle and the mouth of hydra was shown to be a direct measure of the extent of the feeding response.

[23] This process takes place not only in the pieces of tissue excised from the body column, but also from re-aggregates of dissociated single cells.

Active mobility of these endodermal epithelial cells forms two layers in both the re-aggregate and the re-generating tip of the excised tissue.

[25] This publication has been widely cited as evidence that Hydra do not senesce (do not age), and that they are proof of the existence of non-senescing organisms generally.

In 2010, Preston Estep published (also in Experimental Gerontology) a letter to the editor arguing that the Martinez data refutes the hypothesis that Hydra do not senesce.

The transcription factor "forkhead box O" (FoxO) has been identified as a critical driver of the continuous self-renewal of Hydra.

[27] In bilaterally symmetrical organisms (Bilateria), the transcription factor FoxO affects stress response, lifespan, and increase in stem cells.

If this transcription factor is knocked down in bilaterian model organisms, such as fruit flies and nematodes, their lifespan is significantly decreased.

In experiments on H. vulgaris (a radially symmetrical member of phylum Cnidaria), when FoxO levels were decreased, there was a negative effect on many key features of the Hydra, but no death was observed, thus it is believed other factors may contribute to the apparent lack of aging in these creatures.

Their identification in hydra was based, in part, on the presence in its genome of genes homologous to ones present in other genetically well studied species playing key roles in these DNA repair pathways.

This is because the brown hydra genome is the result of an expansion event involving LINEs, a type of transposable elements, in particular, a single family of the CR1 class.

[31] Due to the simplicity of their life cycle when compared to other hydrozoans, hydras have lost many genes that correspond to cell types or metabolic pathways of which the ancestral function is still unknown.

Thanks to this feature, many reporter cell lines have been created with regions around 500 to 2000 bases upstream of the gene of interest.

It presents a great number of intercentromeric interactions when compared to other cnidarians, probably due to the loss of multiple subunits of condensin II.

It is organized in domains that span dozens to hundreds of megabases, containing epigenetically co-regulated genes and flanked by boundaries located within heterochromatin.