Hydra – External Anatomy

Hydras are animals with radially symmetrical bodies that can grow up to 10 mm in length. They attach to substrates using a basal disc, or "foot," which secretes a sticky fluid for adhesion. At the opposite end of their body is an opening that functions as both mouth and anus, located in the center of their tentacles. These tentacles, which are highly elastic, can number up to twelve. When food is abundant, hydras reproduce asexually through a process called strobilation. In this process, a miniature version of the adult hydra grows from the body wall. Once it reaches a certain size, it detaches and becomes an independent organism. This entire strobilation process can occur in as little as two days when food is plentiful. In adverse conditions, such as the approach of winter, hydras are capable of sexual reproduction. The body wall swells, and either testes or ovaries develop within it. In some species, a single hydra may have both male and female reproductive organs. When the adult dies, the fertilized eggs remain dormant in the mud, awaiting favorable conditions to begin a new generation.