The parasitic actinula is found attached to the proboscis of the medusa; it thrusts its greatly elongated hypostome into the mouth of the medusa and nourishes itself upon the food in the digestive cavity of its host.
To become a medusa, the actinula grows scarcely at all in the direction of the principal axis, but greatly along a plane at right angles to it.
The actinula stage is usually suppressed or not set free, but it is seen in Tubularia (fig.
Without a hydroid phase; the medusa develops directly from the actinula larva, which may, however, multiply by budding.
In Cordylophora the embryo is set free at the parenchymula stage as a planula which fixes itself and develops into a polyp, both gastrula and actinula stages being suppressed.
In the spring the embryo bursts its shell and is set free as a minute actinula which becomes a Hydra.
In Gonionemus the actinula becomes fixed and polyp-like, and reproduces by budding, so that here also an alternation of generations may occur.
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.
The actinula, when free, may multiply by larval budding, but in all cases both the original actinula and all its descendants become converted into medusae, so that there is no alter nation of generations.
Thus in Cunina octonaria, the ovum develops into an actinula which buds daughteractinulae; all of them, both parent and offspring, develop into medusae, so that there is no alternation of generations, but only larval multiplication.