Arms, tentacles, and antennae stretched for it.
Was there any place far enough away to escape the tentacles of paternal love?
(iii.) The collar is produced dorsally into arms (one pair in Rhabdopleura, four to eight pairs in Cephalodiscus), each of which bears numerous ciliated tentacles, the organs by which the microscopic food-particles are conveyed to the mouth.
Just below the crown of tentacles, however, the body widens out to form a " head," termed the hydranth (a), containing a stomach-like dilatation of the digestive cavity.
On the upper face of the hydranth the crown of tentacles (t) surrounds the peristome, from which rises the conical hypostome, bearing the mouth at its extremity.
In the Gymnoblastea the perisarc either stops below the hydranth, or, if continued on to it, forms a closely-fitting investment extending as a thin cuticle as far as the bases of the tentacles (e.g.
As regards number, we find in the aberrant forms Protohydra and Microhydra tentacles entirely absent.
II, A) the polyp bears two tentacles only.
The reduction of the tentacles in all these forms may be correlated with their mode of life, and especially with living in a constant current of water, which brings foodparticles always from one direction and renders a complete whorl or circle of tentacles unnecessary.
With the exception of these forms, reduced for the most part in correlation with a semi-parasitic mode of life, the tentacles are usually numerous.
It is rare to find in the polyp a regular, symmetrical disposition of the tentacles as in the medusa.
The tentacles may form a single whorl, or more than one; thus in Corymorpha (fig.
2) several; in Coryne and Cordylophora the tentacles are scattered irregularly over the elongated hydranth.
As regards form, the tentacles show a.number of types, of which the most important are (I) filiform, i.e.
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.
A further stage in evolution is that the muscle-cells lose their connexion with the epithelium and come to lie entirely beneath it, forming a sub-epithelial contractile layer, developed chiefly in the tentacles of the polyp. The of the evolution of the ganglioncells is probably similar; an epithelial cell develops processes of nervous nature from the base, which come into connexion with the bases of the sensory cells, with the muscular cells, and with the similar processes of other nerve-cells; next the nerve-cell loses its connexion with the outer epithelium and becomes a sub-epithelial ganglion-cell which is closely connected with the muscular layer, conveying stimuli from the sensory cells to the contractile elements.