This name we give to those extinct Protozoa which correspond to the two ontogenetic embryonic forms of the six higher animal tribes, namely, the Planula and the Gastrula.
The Planula is very probably the ancestral form of the Coelenterata.
The name Planula signifies "wandering animal," because the Planula larva swims about by means of cilia.
Among the few Cœlenterates which have no free planulastage is the one truly fluviatile genus--Hydra.
The sedentary Anemones pass through a planula stage, but within the body of the parent.
One end of the planula becomes somewhat oval and develops a special bunch of cilia.
A ciliated planula with delaminated hypoblast is also found in Gorgonia and Corallium rubrum.
In the Alcyonidae the segmentation appears always to lead to the formation of a solid morula, which becomes a planula by delamination.
The planula in its primitive condition is not bilaterally symmetrical, but frequently, as amongst the Actinozoa, it becomes flattened on two sides before undergoing its conversion into the adult form.
At this stage the larva has the typical planula form.
The segmentation and formation of a two-layered planula (fig.
The planula occurs in the majority of sessile forms of Hydrozoa except the Tubularidae and Hydra.
We are probably justified in assuming that the planula is a repetition of a free ancestral form of the Coelenterata.
In the Tubularidae and in Hydra an abbreviated development leads no doubt to the absence of a free planula stage, and the absence of a larval form amongst the Ctenophora may, as has already been stated, be probably explained in the same way.
How many of these characters did the ancestral planula possess?
Perhaps the bilateral form ofplanula is the starting point both for the Coelenterata and the Turbellaria.
The eggs are not always laid in the condition of the simpleplanula described above.
Suppose, for instance, that the planula of our Melicertum (See Fig.
At [pg138] the end of July they are fully developed, and begin to discharge their eggs, which go into the folds around the mouth and remain there until they attain the planula stage.
The upper surface of theplanula then becomes depressed and forms a gullet, and in time a complete animal is formed.
Thus from the original planula three appendages are, as it were, budded off, while the planula itself mostly gives rise to coenosarc, just as in some hydroids the planula is converted chiefly into hydrorhiza.
The planula becomes elongated and broader towards one pole, at which a pit or invagination of the ectoderm arises.
This is in some degree parallel to the cases described above, in which a planula gives rise to the hydrorhiza, and buds a polyp laterally.
The planula has its two extremities dissimilar (Bipolaria-larva).
Great apparent differences may also be brought about by variations in the period at which the embryo is set free as a larva, and since two free-swimming stages, planula and actinula, are unnecessary, one or other of them is always suppressed.
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 Aeginopsis a planula is formed by multipolar immigration.
The endoderm of the planulanow acquires a cavity, and at the narrower pole a mouth is formed, giving rise to the primary siphon.
In Leptolinae the embryonic development culminates in a polyp, which is usually formed by fixation of a planula (parenchymula), rarely by fixation of an actinula.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "planula" in a variety of sentences. We hope that you will now be able to make sentences using this word.
