The larva has now become a planula, and consists of a closed sack with double walls.
In Aeginopsis a planula is formed by multipolar immigration.
At this period cilia appear on the surface, and the larva becomes a planula.
The planula has its two extremities dissimilar (Bipolaria-larva).
The one group contains the larva of the Cœlenterata or planula, the other group the larv of all the other forms.
The planula is very probably the ancestral form of the Cœlenterata.
One end of the planula becomes somewhat oval and develops a special bunch of cilia.
planula, plan′ū-la, n. the locomotory embryo of the cœlenterates.
This new individual is at first an active free-swimming larva called a planula, which does not resemble either a medusa or polyp.
The planula (fig. 216) is characterised by its extreme simplicity.