Regeneration in Hydra
You previously know that the Hydra has spectacular regenerative ability. The hydra is a small tubular, two layered fresh water animal computing 20mm in length. Structurally the Hydra has a head or hypostome at the top end, containing a mouth, surrounded by a ring of about six large tentacles that bear a number of stinging cells called cnidoblasts. Towards the posterior end, the hydra has a stalk or peduncle ending in a broad base known as pedal or basal disc or foot. The creature joints itself to the substratum by means of the pedal disc. Close to the base is the budding region from where the asexual buds arise. The stomach or gastric region in which most of the digestion occurs is located between the hypostome and the budding region. The body wall of the Hydra contains two concentric epithelial layers
Figure: Anatomy of Hydra
a) Gastrodermis derived from the endoderm (internal layer), surrounding a central gastric cavity. The epidermis and gastrodermis are separated by an acellular matrix called mesogle.
b) Epidermis derived from the ectoderm (external layer).