Planula

(plan' -u-lah). In biology, the globular or oval, free-swimming, two-layered, solid larva of certain of the Porifer a and Cal enter at a. It is destitute of a mouth or blastopore, and succeeds the blastula in those metazoans in which gastrulation takes place by immigration and delamination, instead of by invagination. The planula is succeeded by the Planogastrula and this in turn by the gastrula stage proper, with its blastopore. Salensky describes a three-layered planula in the Rotator i a and other forms. The term planula was first applied by Dalzell to the motile larvae of certain hydrozoans (" zoophytes "), but it has come to have a broader application. Planula may be regarded as the ancestral form of the Cnidaria. It appears to correspond to the parenchymula of Metschnikoff. ula.