Figure 1.

(a) Picture of living Hydra. The animal has a simple body plan: it is a tube with a head at the apical end and a foot or basal disc at the other. The head is in two parts, the hypostome (mouth) at the apex and below that the tentacle zone from which a ring of tentacles emerge. Scale bar 200 μm. (b) Schematic representation of the bilayered structure of the animal: the body wall is composed of two self-renewing cell layers, an outer, the ectoderm, and an inner, the endoderm, separated by an extracellular matrix, the mesoglea. The arrows on the left side indicate the direction of tissue displacement. (c) Along the animal body, both ectoderm and endoderm layers are composed of epitheliomuscular cells, while interstitial stem cells and their intermediate and terminal derivatives (neurons, nematocytes, and secretory cells) are interspersed among ectoderm and endoderm.