(For other anatomical variations see Hydromedusae and Scyphomedusae.) In development the medusa can be derived easily by a process of differential growth, combined with concrescence of cell-layers, from the actinula-larva.
(For figures see Hydrozoa.) The actinula is polyp-like, with a sack-like or rounded body; a crown of tentacles surrounds a wide peristome, in the centre of which is the mouth, usually raised on a conical process termed the hypostome.
To produce a medusa the actinula grows greatly along a plane at right angles to the vertical axis of the body, whereby the aboral surface of the actinula becomes the exumbrella, and the peristome becomes the subumbrella.
The gastrula has now become an actinula, which may be termed the distinctive larva of the Cnidaria, and doubtless represents in a transitory manner the common ancestor of the group. In no case known, however, does the actinula become the adult, sexually mature individual, but always undergoes further modifications, whereby it develops into either a polyp or a medusa.
- Diagram showing the change of the Actinula (A) into a Polyp (B); a-b, principal (vertical) axis; c-d, horizontal axis.