Finally, as regards structure,S the tentacles may retain their primitive hollow nature, or become solid by obliteration of the axial cavity.
(2) Sensory cells, which may be fairly numerous in places, especially on the tentacles, but which occur always scattered and isolated, never aggregated to form sense-organs as in the medusa.
A third point of dispute is whether the nematocysts ar:e formed in situ, or whether the cnidoblasts migrate with them to the region where they are most needed; the fact that in Hydra, for example, there are no interstitial cells in the tentacles, where nematocysts are very abundant, is certainly in favour of the view that the cnidoblasts migrate on to the tentacles from the body, and that like the genital cells the cnidoblasts are wandering cells.
Dz, by the hydranths, each with dactylozoid; gz, gastrozoid; b, mouth and tentacles; and, blastostyle; gon, gonophores; secondly, the " coenosarc," or rh, hydrorhiza.
The gastrozoids (gz), have the tentacles reduced or absent, but have the mouth and stomach enlarged.